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1.1     ! root        1: .so ../ADM/mac
        !             2: .XX trofftut 223 "A Troff Tutorial"
        !             3: .nr PI .2i
        !             4: .nr PD 2u
        !             5: .de UC
        !             6: \\$3\s-1\\$1\s+1\\$2
        !             7: ..
        !             8: .de BD
        !             9: \&\\$3\f1\\$1\h'-\w'\\$1'u+3u'\\$1\fP\\$2
        !            10: ..
        !            11: .....TM 76-1273-7 39199 39199-11
        !            12: .TL
        !            13: A Troff Tutorial\(dg
        !            14: .AU
        !            15: Brian W. Kernighan
        !            16: .AI
        !            17: .MH
        !            18: .AB
        !            19: .PP
        !            20: .IT Troff
        !            21: is a text-formatting program for typesetters and laser printers.
        !            22: Such devices normally provide multiple fonts,
        !            23: and a substantial number of mathematical symbols
        !            24: and other special characters.
        !            25: Characters can be printed in a range of sizes,
        !            26: and placed anywhere on the page.
        !            27: .PP
        !            28: .IT Troff
        !            29: allows the user full control over fonts,
        !            30: sizes, and character positions,
        !            31: as well as the usual features of a formatter \(em
        !            32: right-margin justification, automatic hyphenation,
        !            33: page titling and numbering, and so on.
        !            34: It also provides macros, arithmetic variables and operations,
        !            35: and conditional testing, for complicated formatting tasks.
        !            36: .PP
        !            37: This document is an introduction to the most basic use of
        !            38: .IT troff .
        !            39: It presents just enough information to enable the user
        !            40: to do simple formatting
        !            41: tasks like making viewgraphs,
        !            42: and to make incremental changes to existing packages
        !            43: of
        !            44: .IT troff
        !            45: commands.
        !            46: .AE
        !            47: .2C
        !            48: .NH
        !            49: Introduction
        !            50: .tr ^.
        !            51: .PP
        !            52: .FS
        !            53: \(dg This is a version of |reference(troff tutorial v7man) revised by B. W. Kernighan.
        !            54: .FE
        !            55: .IT Troff
        !            56: is a text-formatting program,
        !            57: written by J. F. Ossanna,
        !            58: for producing
        !            59: high-quality printed output from phototypesetters and laser printers.
        !            60: This document is an example of
        !            61: .IT troff
        !            62: output.
        !            63: .PP
        !            64: The single most important rule
        !            65: of using
        !            66: .IT troff
        !            67: is
        !            68: not to use it directly, but through some intermediary.
        !            69: In many ways,
        !            70: .IT troff
        !            71: resembles an assembly language \(em
        !            72: a remarkably powerful and flexible one \(em
        !            73: but nonetheless such that many operations must be specified
        !            74: at a level of detail and in a form that is too hard 
        !            75: for most people to use effectively.
        !            76: .PP
        !            77: For some special applications, there are programs that provide
        !            78: an interface to
        !            79: .IT troff
        !            80: for the majority of users.
        !            81: For example,
        !            82: .IT eqn 
        !            83: |reference(latest eqn)
        !            84: provides an easy to learn language for typesetting mathematics;
        !            85: the 
        !            86: .IT eqn
        !            87: user
        !            88: need know no 
        !            89: .IT troff
        !            90: whatsoever
        !            91: to typeset mathematics.
        !            92: .IT Tbl
        !            93: |reference(latest tbl)
        !            94: provides the same convenience for producing tables.
        !            95: .PP
        !            96: For producing straight text (which may well contain mathematics or tables), there are a number of ``macro packages''
        !            97: that define formatting rules and operations for specific styles
        !            98: of documents,
        !            99: and reduce the amount of
        !           100: direct contact with 
        !           101: .IT troff .
        !           102: In particular, the
        !           103: .CW -ms ,
        !           104: .CW -mpm
        !           105: and
        !           106: .CW -mm
        !           107: packages
        !           108: for Bell Labs internal memoranda and external papers
        !           109: provide most of the facilities needed
        !           110: for a wide range of document preparation.
        !           111: (This paper was prepared with
        !           112: .CW -mpm .)
        !           113: There are also packages for viewgraphs,
        !           114: various journals,
        !           115: and other special applications.
        !           116: Typically you will find these packages easier to use
        !           117: than
        !           118: .IT troff
        !           119: once you get beyond the most trivial operations;
        !           120: you should always consider them first.
        !           121: .PP
        !           122: In the few cases where existing packages don't do the whole job,
        !           123: the solution is
        !           124: .ul
        !           125: not
        !           126: to write an entirely new set of
        !           127: .IT troff
        !           128: instructions from scratch, but to make small changes
        !           129: to adapt packages that already exist.
        !           130: .PP
        !           131: In accordance with this philosophy of letting someone else
        !           132: do the work,
        !           133: the part of
        !           134: .IT troff
        !           135: described here is only a small part of the whole,
        !           136: although it tries to concentrate on the more useful parts.
        !           137: In any case, there is no attempt to be complete.
        !           138: Rather, the emphasis is on showing how to do simple things,
        !           139: and how to make incremental changes to what already exists.
        !           140: The contents of the remaining sections are:
        !           141: .SP .5
        !           142: .nf
        !           143: .in .1i
        !           144: .ta .3i
        !           145: \02.   Point sizes; line spacing
        !           146: \03.   Fonts and special characters
        !           147: \04.   Indents and line lengths
        !           148: \05.   Tabs
        !           149: \06.   Local motions: drawing lines and characters
        !           150: \07.   Strings
        !           151: \08.   Introduction to macros
        !           152: \09.   Titles, pages and numbering
        !           153: 10.    Number registers and arithmetic
        !           154: 11.    Macros with arguments
        !           155: 12.    Conditionals
        !           156: 13.    Environments
        !           157: 14.    Diversions
        !           158:        Appendix A:  Character set
        !           159: .SP .5
        !           160: .in 0
        !           161: .fi
        !           162: .PP
        !           163: To use
        !           164: .IT troff
        !           165: you have to prepare not only the actual text you want printed,
        !           166: but some information that tells
        !           167: .ul
        !           168: how
        !           169: you want it printed.
        !           170: For
        !           171: .IT troff
        !           172: the text
        !           173: and
        !           174: the formatting information are often intertwined quite intimately.
        !           175: Most commands to
        !           176: .IT troff
        !           177: are placed on a line separate from the text itself,
        !           178: beginning with a period (one command per line).
        !           179: For example,
        !           180: .P1
        !           181: Some text.
        !           182: ^ps 14
        !           183: Some more text.
        !           184: .P2
        !           185: will change the `point size',
        !           186: that is,
        !           187: the size of the letters being printed,
        !           188: to `14 point' (one point is 1/72 inch) like this:
        !           189: .P1
        !           190: .ft 1
        !           191: .fi
        !           192: Some text.
        !           193: .ps 14
        !           194: Some more text.
        !           195: .ps 10
        !           196: .P2
        !           197: .PP
        !           198: Occasionally, though,
        !           199: something special occurs in the middle of a line \(em
        !           200: to produce
        !           201: .P1
        !           202: .ft 1
        !           203: Area = \(*p\fIr\fR\|\s8\u2\d\s0
        !           204: .P2
        !           205: you have to type
        !           206: .P1
        !           207: Area = \e(*p\efIr\efR\e\^|\^\es8\eu2\ed\es0
        !           208: .P2
        !           209: (which we will explain shortly).
        !           210: The backslash character
        !           211: .CW  \e 
        !           212: is used 
        !           213: to introduce
        !           214: .IT troff
        !           215: commands and special characters within a line of text.
        !           216: .NH
        !           217: Point Sizes; Line Spacing
        !           218: .PP
        !           219: As mentioned above,
        !           220: the command
        !           221: .CW .ps
        !           222: sets the point size.
        !           223: One point is 1/72 inch,
        !           224: so 6-point characters are at most 1/12 inch high,
        !           225: and 36-point characters are \(12 inch.
        !           226: Different devices provide different sets of sizes;
        !           227: a representative set is shown below.
        !           228: .P1 1
        !           229: .ft 1
        !           230: .ps 6
        !           231: 6 point: Pack my box with five dozen liquor jugs.
        !           232: .ps 7
        !           233: .vs 8p
        !           234: 7 point: Pack my box with five dozen liquor jugs.
        !           235: .vs 9p
        !           236: .ps 8
        !           237: 8 point: Pack my box with five dozen liquor jugs.
        !           238: .vs 10p
        !           239: .ps 9
        !           240: 9 point: Pack my box with five dozen liquor jugs.
        !           241: .vs 11p
        !           242: .ps 10
        !           243: 10 point: Pack my box with five dozen liquor
        !           244: .vs 12p
        !           245: .ps 11
        !           246: 11 point: Pack my box with five dozen 
        !           247: .vs 14p
        !           248: .ps 12
        !           249: 12 point: Pack my box with five dozen
        !           250: .vs 16p
        !           251: .ps 14
        !           252: 14 point: Pack my box with five
        !           253: .vs 24p
        !           254: \s1616 point\s18 18 point\s20 20 point
        !           255: .vs 40p
        !           256: \s2222\s24 24\s28 28\s36 36
        !           257: .ps 10
        !           258: .vs 12p
        !           259: .P2
        !           260: .PP
        !           261: If the number after
        !           262: .CW .ps 
        !           263: is not a
        !           264: legal size,
        !           265: it is rounded up to the next valid value.
        !           266: If no number follows
        !           267: .CW .ps ,
        !           268: .IT troff
        !           269: reverts to the previous size, whatever it was.
        !           270: .IT troff
        !           271: begins with point size 10,
        !           272: which is usually fine.
        !           273: This document is printed in \n(.s point.
        !           274: .PP
        !           275: The point size can also be changed in the middle of a line
        !           276: or even a word
        !           277: with the in-line command
        !           278: .CW \es .
        !           279: To produce
        !           280: .P1
        !           281: .ft 1
        !           282: \s8UNIX\s10 ran on a \s8PDP-\s1011/45
        !           283: .P2
        !           284: type
        !           285: .P1 0
        !           286: \es8UNIX\es10 ran on a \es8PDP-\es1011/45
        !           287: .P2
        !           288: As above,
        !           289: .CW \es 
        !           290: should be followed by a legal point size,
        !           291: except that
        !           292: .CW \es0 
        !           293: causes the size to revert to
        !           294: its previous value.
        !           295: Notice that
        !           296: .CW \es1011
        !           297: can be understood correctly as `size 10, followed by an 11', if the size is legal,
        !           298: but not otherwise.
        !           299: Be cautious with similar constructions.
        !           300: .PP
        !           301: Relative size changes are also legal and useful:
        !           302: .P1
        !           303: \es-2UNIX\es+2
        !           304: .P2
        !           305: temporarily decreases the size, whatever it is, by two points, then
        !           306: restores it, producing \s-2UNIX\s+2.
        !           307: Relative size changes have the advantage that the size difference
        !           308: is independent of the starting size of the document.
        !           309: The amount of the relative change is restricted
        !           310: to a single digit.
        !           311: .PP
        !           312: The other parameter that determines what the type looks like
        !           313: is the spacing between lines,
        !           314: which is set independently of the point size.
        !           315: Vertical spacing is measured from the bottom of one line to
        !           316: the bottom of the next.
        !           317: The command to control vertical spacing is
        !           318: .CW .vs .
        !           319: For running text, it is usually best to set the vertical spacing
        !           320: about 20% bigger than the character size.
        !           321: For example, so far in this document, we have used
        !           322: ``10 on 12'', that is,
        !           323: .P1
        !           324: ^ps 10
        !           325: ^vs 12p
        !           326: .P2
        !           327: If we changed to
        !           328: .P1
        !           329: ^ps 10
        !           330: ^vs 10p
        !           331: .P2
        !           332: .vs 10p
        !           333: .ne 3
        !           334: the running text would look like this.
        !           335: After a few lines, you will agree it looks a little cramped.
        !           336: The right vertical spacing is partly a matter of taste, depending on how
        !           337: much text you want to squeeze into a given space,
        !           338: and partly a matter of traditional printing style.
        !           339: By default,
        !           340: .IT troff
        !           341: uses 10 on 12.
        !           342: .PP
        !           343: .vs 14p
        !           344: .ps 12
        !           345: Point size and vertical spacing make a substantial difference in the amount of text
        !           346: per square inch.
        !           347: This is 12 on 14.
        !           348: .ne 2
        !           349: .PP
        !           350: .ne 2
        !           351: .ps 6
        !           352: .vs 7p
        !           353: Point size and vertical spacing make a substantial difference in the amount of text
        !           354: per square inch.
        !           355: For example,
        !           356: 10 on 12 uses about twice as much space as 7 on 8.
        !           357: This is 6 on 7, which is even smaller.
        !           358: It packs a lot more words per line,
        !           359: but you can go blind trying to read it.
        !           360: .PP
        !           361: When used without arguments,
        !           362: .CW .ps
        !           363: and
        !           364: .CW .vs
        !           365: revert to the previous size and vertical spacing
        !           366: respectively.
        !           367: .PP
        !           368: The command
        !           369: .CW .sp
        !           370: is used to get extra vertical space.
        !           371: Unadorned, 
        !           372: it gives you one extra blank line (one
        !           373: .CW .vs ,
        !           374: whatever that has been set to).
        !           375: Typically, that's more or less than you want,
        !           376: so
        !           377: .CW .sp
        !           378: can be followed by
        !           379: information about how much space you want \(em
        !           380: .P1
        !           381: ^sp 2i
        !           382: .P2
        !           383: means `two inches of vertical space'.
        !           384: .P1
        !           385: ^sp 2p
        !           386: .P2
        !           387: means `two points of vertical space';
        !           388: and
        !           389: .P1
        !           390: ^sp 2
        !           391: .P2
        !           392: means `two vertical spaces' \(em two of whatever
        !           393: .CW .vs
        !           394: is set to
        !           395: (this can also be made explicit with
        !           396: .CW .sp\ 2v );
        !           397: .IT troff
        !           398: also understands decimal fractions in most places,
        !           399: so
        !           400: .P1
        !           401: ^sp 1.5i
        !           402: .P2
        !           403: is a space of 1.5 inches.
        !           404: These same scale factors can be used after
        !           405: .CW .vs
        !           406: to define line spacing, and in fact after most commands
        !           407: that deal with physical dimensions.
        !           408: .PP
        !           409: It should be noted that all size numbers are converted internally
        !           410: to `machine units', which are much smaller, reflecting the finer
        !           411: resolution of particular output devices.
        !           412: Many laser printers run at 300 units/inch;
        !           413: typesetters are often more than 1000 units/inch.
        !           414: .NH
        !           415: Fonts and Special Characters
        !           416: .PP
        !           417: .IT Troff
        !           418: allows many different fonts at any one time.
        !           419: Normally a family of serif fonts (Times roman, italic and bold),
        !           420: a family of san-serif fonts (Helvetica), and a collection of special characters
        !           421: are available.
        !           422: .P1 .2i
        !           423: .ft R
        !           424: abcdefghijklmnopqrstuvwxyz 0123456789
        !           425: ABCDEFGHIJKLMNOPQRSTUVWXYZ
        !           426: .ft I
        !           427: abcdefghijklmnopqrstuvwxyz 0123456789
        !           428: ABCDEFGHIJKLMNOPQRSTUVWXYZ
        !           429: .ft B
        !           430: abcdefghijklmnopqrstuvwxyz 0123456789
        !           431: ABCDEFGHIJKLMNOPQRSTUVWXYZ
        !           432: .ft BI
        !           433: abcdefghijklmnopqrstuvwxyz 0123456789
        !           434: ABCDEFGHIJKLMNOPQRSTUVWXYZ
        !           435: .sp 3p
        !           436: .ft H
        !           437: abcdefghijklmnopqrstuvwxyz 0123456789
        !           438: ABCDEFGHIJKLMNOPQRSTUVWXYZ
        !           439: .ft HI
        !           440: abcdefghijklmnopqrstuvwxyz 0123456789
        !           441: ABCDEFGHIJKLMNOPQRSTUVWXYZ
        !           442: .ft HB
        !           443: abcdefghijklmnopqrstuvwxyz 0123456789
        !           444: ABCDEFGHIJKLMNOPQRSTUVWXYZ
        !           445: .P2
        !           446: A representative collection of
        !           447: greek, mathematical symbols and miscellaneous other characters are
        !           448: listed in Appendix A.
        !           449: Appendix B shows the font families commonly available on Postscript printers;
        !           450: your mileage may vary.
        !           451: .PP
        !           452: .IT Troff
        !           453: prints in roman unless told otherwise.
        !           454: To switch into bold, use
        !           455: the
        !           456: .CW .ft
        !           457: command
        !           458: .P1
        !           459: ^ft B
        !           460: .P2
        !           461: and for italics,
        !           462: .P1
        !           463: ^ft I
        !           464: .P2
        !           465: To return to roman, use
        !           466: .CW .ft\ R ;
        !           467: to return to the previous font,
        !           468: whatever it was,
        !           469: use either
        !           470: .CW .ft\ P
        !           471: or just
        !           472: .CW .ft  .
        !           473: The `underline' command
        !           474: .P1
        !           475: ^ul
        !           476: .P2
        !           477: causes the next input line to print in italics.
        !           478: .CW .ul
        !           479: can be followed by a count to
        !           480: indicate that more than one line is to be italicized.
        !           481: .PP
        !           482: Fonts can also be changed within a line or word
        !           483: with the in-line command
        !           484: .CW \ef :
        !           485: .P1
        !           486: \fBbold\fIface\fR text
        !           487: .P2
        !           488: is produced by
        !           489: .P1
        !           490: \efBbold\efIface\efR text
        !           491: .P2
        !           492: If you want to do this so the previous font, whatever it was,
        !           493: is left undisturbed, insert extra
        !           494: .CW \efP
        !           495: commands, like this:
        !           496: .P1
        !           497: \efBbold\efP\efIface\efP\efR text\efP
        !           498: .P2
        !           499: Because only the immediately previous font is remembered,
        !           500: you have to restore the previous font after each change
        !           501: or you can lose it.
        !           502: The same is true of 
        !           503: .CW .ps
        !           504: and
        !           505: .CW .vs
        !           506: when used without an argument.
        !           507: .PP
        !           508: There are many other fonts available besides the standard set.
        !           509: Normally you can just use a font by naming it,
        !           510: but if an entire document is to be printed
        !           511: in a non-standard set of fonts,
        !           512: it is best to mount the fonts explicitly.
        !           513: This tells 
        !           514: .IT troff
        !           515: which fonts you will make the most use of.
        !           516: The
        !           517: .CW .fp
        !           518: command
        !           519: .P1
        !           520: ^fp 3 PB
        !           521: .P2
        !           522: says that the Palatino Bold font
        !           523: .ft PB
        !           524: (which looks like this)
        !           525: .ft
        !           526: is mounted on position 3.
        !           527: .PP
        !           528: It is possible to make a document relatively independent
        !           529: of the actual fonts used to print it
        !           530: by using font numbers instead of names;
        !           531: for example,
        !           532: .CW \ef3
        !           533: and
        !           534: .CW ".ft 3
        !           535: mean `whatever font is mounted at position 3',
        !           536: and thus work for any setting.
        !           537: The standard macro packages assume that fonts 1 through 4 contain
        !           538: roman, italic, bold, and bold italic members of a family.
        !           539: .PP
        !           540: There is also a way to get synthetic bold fonts
        !           541: by overstriking letters with a slight offset,
        !           542: with the
        !           543: .CW .bd
        !           544: command;
        !           545: it is illustrated in Section 11.
        !           546: .PP
        !           547: Special characters have four-character names beginning with
        !           548: .CW \e( ;
        !           549: they may be inserted anywhere.
        !           550: For example,
        !           551: .P1
        !           552: .ft 1
        !           553: \(14 + \(12 = \(34
        !           554: .P2
        !           555: is produced by
        !           556: .P1
        !           557: \e(14 + \e(12 = \e(34
        !           558: .P2
        !           559: In particular,
        !           560: greek letters are all of the form
        !           561: .CW  \e(*\- ,
        !           562: where
        !           563: .CW -
        !           564: is an upper or lower case roman letter
        !           565: reminiscent of the greek.
        !           566: Thus
        !           567: to get
        !           568: .P1
        !           569: .ft 1
        !           570: \(*S(\(*a\(mu\(*b) \(-> \(if
        !           571: .P2
        !           572: in bare
        !           573: .IT troff
        !           574: we have to type
        !           575: .P1
        !           576: \e(*S(\e(*a\e(mu\e(*b) \e(\(mi> \e(if
        !           577: .P2
        !           578: That line is unscrambled as follows:
        !           579: .P1
        !           580: .ta 1i 2i 3i
        !           581: \e(*S  \(*S
        !           582: (      (
        !           583: \e(*a  \(*a
        !           584: \e(mu  \(mu
        !           585: \e(*b  \(*b
        !           586: )      )
        !           587: \e(\(mi>       \(->
        !           588: \e(if  \(if
        !           589: .P2
        !           590: Appendix A contains a sample of special names.
        !           591: .PP
        !           592: In
        !           593: .IT eqn 
        !           594: the same effect can be achieved with the input
        !           595: .P1
        !           596: SIGMA ( alpha times beta ) \-> inf
        !           597: .P2
        !           598: which is less concise, but clearer to the uninitiated.
        !           599: .PP
        !           600: Notice that
        !           601: each
        !           602: four-character name is a single character
        !           603: as far as
        !           604: .IT troff
        !           605: is concerned \(em
        !           606: the
        !           607: `translate' command
        !           608: .P1
        !           609: ^tr \e(mi\e(em
        !           610: .P2
        !           611: is perfectly clear, meaning
        !           612: .P1
        !           613: ^tr \f1\(mi\h'.1m'\(em
        !           614: .P2
        !           615: that is, to translate \(mi into \(em.
        !           616: .PP
        !           617: Some characters are automatically translated into others:
        !           618: grave  \(ga  and acute  \(aa  accents (apostrophes) become open and close single quotes
        !           619: `\0';
        !           620: the combination of ``...'' is generally preferable to the double quotes "...".
        !           621: Similarly a typed minus sign becomes a hyphen -.
        !           622: To print an explicit \- sign, use
        !           623: .CW \e- .
        !           624: To print a backslash, use
        !           625: .CW \ee .
        !           626: .NH
        !           627: Indents and Line Lengths
        !           628: .PP
        !           629: .IT Troff
        !           630: starts with a line length of 6.5 inches,
        !           631: too wide for 8\(12\(mu11 paper.
        !           632: To reset the line length,
        !           633: use
        !           634: the
        !           635: .CW .ll
        !           636: command:
        !           637: .P1
        !           638: ^ll 6i
        !           639: .P2
        !           640: As with
        !           641: .CW .sp ,
        !           642: the actual length can be specified in several ways;
        !           643: inches are probably the most intuitive.
        !           644: .PP
        !           645: Output is normally positioned an inch from the left edge of the paper.
        !           646: To reset the default physical left margin (``page offset''),
        !           647: use the
        !           648: .CW .po
        !           649: command.
        !           650: .P1
        !           651: ^po 0
        !           652: .P2
        !           653: sets the offset as far to the left as it will go.
        !           654: .PP
        !           655: The indent command
        !           656: .CW .in
        !           657: causes the left margin to be indented
        !           658: by some specified amount from the page offset.
        !           659: If we use
        !           660: .CW .in
        !           661: to move the left margin in,
        !           662: and
        !           663: .CW .ll
        !           664: to move the right margin to the left,
        !           665: we can
        !           666: make offset blocks of text:
        !           667: .P1
        !           668: ^in 0.3i
        !           669: ^ll \(mi0.3i
        !           670: text to be set into a block
        !           671: ^ll +0.3i
        !           672: ^in \(mi0.3i
        !           673: .P2
        !           674: will create a block that looks like this:
        !           675: .P1
        !           676: .fi
        !           677: .ll -0.3i
        !           678: .ft 1
        !           679: Pater noster qui est in caelis sanctificetur nomen tuum;
        !           680: adveniat regnum tuum; fiat voluntas tua, sicut in caelo,
        !           681: et in terra. ...
        !           682: Amen.
        !           683: .ll +0.3i
        !           684: .P2
        !           685: Notice the use of `+' and `\(mi'
        !           686: to specify the amount of change.
        !           687: These change the previous setting by the specified amount,
        !           688: rather than just overriding it.
        !           689: The distinction is quite important:
        !           690: .CW .ll\ +1i
        !           691: makes lines one inch longer;
        !           692: .CW .ll\ 1i
        !           693: makes them one inch
        !           694: .ul
        !           695: long.
        !           696: .PP
        !           697: With 
        !           698: .CW .in ,
        !           699: .CW .ll
        !           700: and
        !           701: .CW .po ,
        !           702: the previous value is used if no argument is specified.
        !           703: .PP
        !           704: To indent a single line, use the `temporary indent'
        !           705: command
        !           706: .CW .ti .
        !           707: For example, all paragraphs in this memo
        !           708: effectively begin with the command
        !           709: .P1
        !           710: ^ti 3
        !           711: .P2
        !           712: Three of what?
        !           713: The default unit for
        !           714: .CW .ti ,
        !           715: as for most horizontally oriented commands
        !           716: .CW .ll , (
        !           717: .CW .in ,
        !           718: .CW .po ),
        !           719: is ems;
        !           720: an em is roughly the width of the letter `m'
        !           721: in the current point size.
        !           722: (Precisely, a em in size
        !           723: .ul
        !           724: p
        !           725: is
        !           726: .ul
        !           727: p
        !           728: points.)
        !           729: Although inches are usually clearer than ems to people who don't set type
        !           730: for a living,
        !           731: ems have a place:
        !           732: they are a measure of size that is proportional to the current point size.
        !           733: If you want to make text that keeps its proportions
        !           734: regardless of point size,
        !           735: you should use ems for all dimensions.
        !           736: Ems can be specified as scale factors directly,
        !           737: as in
        !           738: .CW .ti\ 2.5m .
        !           739: .PP
        !           740: Lines can also be indented negatively
        !           741: if the indent is already positive:
        !           742: .P1
        !           743: ^ti \(mi0.3i
        !           744: .P2
        !           745: causes the next line to be moved back three tenths of an inch.
        !           746: Thus to make a decorative initial capital,
        !           747: we indent the whole paragraph, then move the letter `P' back with
        !           748: a
        !           749: .CW .ti
        !           750: command:
        !           751: .KS
        !           752: .SP .5
        !           753: .ll -0.3i
        !           754: .fi
        !           755: .in +.6i
        !           756: .ti -0.3i
        !           757: .ft 1
        !           758: \s36\v'2'P\v'-2'\s0ater noster qui est in caelis sanctificetur
        !           759: nomen tuum;
        !           760: adveniat regnum tuum;
        !           761: 'in -.3i
        !           762: fiat voluntas tua,
        !           763: sicut in caelo, et in terra. ...
        !           764: Amen.
        !           765: .ll +0.3i
        !           766: .in 0
        !           767: .br
        !           768: .SP .5
        !           769: .KE
        !           770: .LP
        !           771: Of course, there is also some trickery to make the `P'
        !           772: bigger (just a
        !           773: .CW \es36P\es0 ),
        !           774: and to move it
        !           775: down from its normal position
        !           776: (see the section on local motions).
        !           777: .NH
        !           778: Tabs
        !           779: .PP
        !           780: Tabs
        !           781: (the \s8ASCII\s0 `horizontal tab' character)
        !           782: can be used to produce output in columns,
        !           783: or to set the horizontal position of output.
        !           784: Typically
        !           785: tabs are used only in unfilled text.
        !           786: Tab stops are set by default every half inch from the
        !           787: current indent,
        !           788: but
        !           789: can be changed by the
        !           790: .CW .ta
        !           791: command.
        !           792: To set stops every inch, for example,
        !           793: .P1
        !           794: ^ta 1i 2i 3i 4i 5i 6i
        !           795: .P2
        !           796: .PP
        !           797: By default the stops are left-justified
        !           798: (as on a typewriter),
        !           799: but there are also centering and right-adjusting tabs.
        !           800: .PP
        !           801: For a handful of numeric columns, use right-adjusting tabs:
        !           802: .P1
        !           803: ^nf
        !           804: ^ta .5iR 1iR 1.5iR
        !           805: \fItab\f(CW 1\fI tab \f(CW2\fI tab \f(CW3
        !           806: \fItab \f(CW40\fI tab \f(CW50\fI tab \f(CW60
        !           807: \fItab \f(CW700\fI tab \f(CW800\fI tab \f(CW900\f(CW
        !           808: ^fi
        !           809: .P2
        !           810: The 
        !           811: .CW R
        !           812: means ``right adjust.''
        !           813: When printed, this will produce
        !           814: .P1 0
        !           815: .ft 1
        !           816: .ta .5iR 1iR 1.5iR
        !           817:        1       2       3
        !           818:        40      50      60
        !           819:        700     800     900
        !           820: .P2
        !           821: If you have many numbers,
        !           822: or if you need more complicated table layout,
        !           823: don't
        !           824: use
        !           825: .IT troff 
        !           826: directly;
        !           827: use
        !           828: .IT tbl .
        !           829: .PP
        !           830: It is also possible to fill up tabbed-over space with
        !           831: some character other than blanks by setting the `tab replacement character'
        !           832: with the
        !           833: .CW .tc
        !           834: command:
        !           835: .P1
        !           836: ^ta 1.5i 2.5i
        !           837: ^tc \e(ru      (\e(ru is "\(ru")
        !           838: Name \fItab\fR Age \fItab\fR 
        !           839: .P2
        !           840: produces
        !           841: .P1 3
        !           842: .ft 1
        !           843: .ta 1.5i 2.5i
        !           844: .tc \(ru
        !           845: Name    Age    
        !           846: .tc
        !           847: .P2
        !           848: To reset the tab replacement character to a blank, use
        !           849: .CW .tc
        !           850: with no argument.
        !           851: (Lines can also be drawn with the
        !           852: .CW \el
        !           853: command, described in Section 6.)
        !           854: .PP
        !           855: .IT Troff
        !           856: also provides a very general mechanism called `fields'
        !           857: for setting up complicated columns.
        !           858: (This is used by
        !           859: .IT tbl .)
        !           860: We will not go into it in this paper.
        !           861: .NH
        !           862: Local Motions: Drawing lines and characters
        !           863: .PP
        !           864: .sp 10p
        !           865: Remember `Area = \(*pr\u2\d' and the big `P'
        !           866: in the Paternoster.
        !           867: How are they done?
        !           868: .IT Troff
        !           869: provides a host of commands for placing characters of any size
        !           870: at any place.
        !           871: You can use them to draw special characters 
        !           872: or to tune your output for a particular appearance.
        !           873: Most of these commands are straightforward, but messy to read
        !           874: and tough to type correctly.
        !           875: .PP
        !           876: If you won't use 
        !           877: .IT eqn ,
        !           878: subscripts and superscripts are most easily done with
        !           879: the half-line local motions
        !           880: .CW \eu
        !           881: and
        !           882: .CW \ed .
        !           883: To go back up the page half a point-size, insert a
        !           884: .CW \eu
        !           885: at the desired place;
        !           886: to go down, insert a
        !           887: .CW \ed .
        !           888: .CW \eu \& (
        !           889: and
        !           890: .CW \ed
        !           891: should always
        !           892: be used in pairs, as explained below.)
        !           893: Thus
        !           894: .P1
        !           895: Area = \e(*pr\eu2\ed
        !           896: .P2
        !           897: produces
        !           898: .P1
        !           899: .ft 1
        !           900: Area = \(*pr\u2\d
        !           901: .P2
        !           902: To make the `2' smaller, bracket it with
        !           903: .CW \es\-2
        !           904: and
        !           905: .CW \es0 .
        !           906: Since
        !           907: .CW \eu
        !           908: and
        !           909: .CW \ed
        !           910: refer to the current point size,
        !           911: be sure to put them either both inside or both outside
        !           912: the size changes,
        !           913: or you will get an unbalanced vertical motion.
        !           914: .PP
        !           915: Sometimes the space given by
        !           916: .CW \eu
        !           917: and
        !           918: .CW \ed
        !           919: isn't the right amount.
        !           920: The 
        !           921: .CW \ev
        !           922: command can be used to request an arbitrary amount of vertical motion.
        !           923: The in-line command
        !           924: .P1
        !           925: \ev'\f2amount\fP'
        !           926: .P2
        !           927: causes motion up or down the page by the amount specified in
        !           928: `\f2amount\fP'.
        !           929: For example, to move the `P' down, we used
        !           930: .P1 2
        !           931: .ta 1i
        !           932: ^in +0.6i      (move paragraph in)
        !           933: ^ll \-0.3i     (shorten lines)
        !           934: ^ti \-0.3i     (move P back)
        !           935: \ev'2'\es36P\es0\ev'\-2'ater noster qui est
        !           936: in caelis ...
        !           937: .P2
        !           938: A minus sign causes upward motion, while
        !           939: no sign or a plus sign means down the page.
        !           940: Thus
        !           941: .CW \ev'\-2'
        !           942: causes an upward vertical motion 
        !           943: of two line spaces.
        !           944: .PP
        !           945: There are many other ways to specify the amount of motion \(em
        !           946: .P1
        !           947: \ev'0.1i'
        !           948: \ev'3p'
        !           949: \ev'\-0.5m'
        !           950: .P2
        !           951: and so on are all legal.
        !           952: Notice that the scale specifier
        !           953: .CW i
        !           954: or
        !           955: .CW p
        !           956: or
        !           957: .CW m
        !           958: goes inside the quotes.
        !           959: Any character can be used in place of the quotes;
        !           960: this is also true of all other
        !           961: .IT troff
        !           962: commands described in this section.
        !           963: .PP
        !           964: Since
        !           965: .IT troff
        !           966: does not take within-the-line vertical motions into account
        !           967: when figuring out where it is on the page,
        !           968: output lines can have unexpected positions
        !           969: if the left and right ends aren't at the same
        !           970: vertical position.
        !           971: Thus
        !           972: .CW \ev ,
        !           973: like
        !           974: .CW \eu
        !           975: and
        !           976: .CW \ed ,
        !           977: should always balance upward vertical motion in a line with
        !           978: the same amount in the downward direction.
        !           979: .PP
        !           980: Arbitrary horizontal motions are also available \(em
        !           981: .CW \eh
        !           982: is like
        !           983: .CW \ev ,
        !           984: except that the default scale factor is ems instead of line spaces.
        !           985: As an example,
        !           986: .P1
        !           987: \eh'\-0.1i'
        !           988: .P2
        !           989: causes a backwards motion of a tenth of an inch.
        !           990: Consider printing the mathematical symbol
        !           991: `>>'.
        !           992: The default spacing is a bit too wide, so
        !           993: .IT eqn
        !           994: replaces this by
        !           995: .P1
        !           996: >\eh'\-0.1m'>
        !           997: .P2
        !           998: to produce >\h'-.1m'>.
        !           999: .PP
        !          1000: Frequently
        !          1001: .CW \eh
        !          1002: is used with the `width function'
        !          1003: .CW \ew
        !          1004: to generate motions equal to the width
        !          1005: of some character string.
        !          1006: The construction
        !          1007: .P1
        !          1008: \ew'thing'
        !          1009: .P2
        !          1010: is a number equal to the width of `thing' in machine units.
        !          1011: All
        !          1012: .IT troff
        !          1013: computations are ultimately done in these units.
        !          1014: To move horizontally the width of an `x',
        !          1015: we can say
        !          1016: .P1
        !          1017: \eh'\ew'x'u'
        !          1018: .P2
        !          1019: As we mentioned above,
        !          1020: the default scale factor for
        !          1021: all horizontal dimensions is
        !          1022: .CW m ,
        !          1023: ems, so here we must have the
        !          1024: .CW u
        !          1025: for machine units,
        !          1026: or the motion produced will be far too large.
        !          1027: .IT Troff
        !          1028: is quite happy with the nested quotes, by the way,
        !          1029: so long as you don't leave any out.
        !          1030: .SP
        !          1031: .PP
        !          1032: There are also several special-purpose
        !          1033: .IT troff
        !          1034: commands for local motion.
        !          1035: .CW \e0
        !          1036: is an unpaddable white space
        !          1037: of the same width as a digit.
        !          1038: `Unpaddable' means that it will never be widened
        !          1039: or split across a line by line justification and filling.
        !          1040: There is also
        !          1041: .CW \e (blank),
        !          1042: .tr ^^
        !          1043: which is an unpaddable character the width of a space,
        !          1044: .CW \e| ,
        !          1045: which is half that width,
        !          1046: .CW \e^ ,
        !          1047: which is one quarter of the width of a space,
        !          1048: and
        !          1049: .CW \e& ,
        !          1050: which has zero width.
        !          1051: .tr ^.
        !          1052: (This last one is useful, for example, in entering
        !          1053: a text line which would otherwise begin with a `.'.)
        !          1054: .PP
        !          1055: The command
        !          1056: .CW \eo ,
        !          1057: used like
        !          1058: .P1
        !          1059: \eo'set of characters'
        !          1060: .P2
        !          1061: causes (up to 9)
        !          1062: characters to be overstruck,
        !          1063: centered on the widest.
        !          1064: This is nice for accents, as in
        !          1065: .P1
        !          1066: m\eo"e\e(aa"long\eo"e\e(ga"ne
        !          1067: .P2
        !          1068: which makes
        !          1069: .P1
        !          1070: .ft 1
        !          1071: m\o"e\(aa"long\o"e\(ga"ne
        !          1072: .P2
        !          1073: The accents are
        !          1074: .CW \e(ga
        !          1075: and
        !          1076: .CW \e(aa ,
        !          1077: or
        !          1078: .CW \e\` 
        !          1079: and
        !          1080: .CW \e\' ;
        !          1081: remember that each is just one character to
        !          1082: .IT troff .
        !          1083: .SP
        !          1084: .PP
        !          1085: You can make your own overstrikes with another special convention,
        !          1086: .CW \ez ,
        !          1087: the zero-motion command.
        !          1088: .CW \ezx
        !          1089: suppresses the normal horizontal motion
        !          1090: after printing the single character
        !          1091: .CW x ,
        !          1092: so another character can be laid on top of it.
        !          1093: Although sizes can be changed within
        !          1094: .CW \eo ,
        !          1095: it centers the characters on the widest,
        !          1096: and
        !          1097: there can be no horizontal or vertical motions,
        !          1098: so
        !          1099: .CW \ez
        !          1100: may be the only way to get what you want:
        !          1101: .P1
        !          1102: .sp 2
        !          1103: \s8\z\(sq\s14\z\(sq\s22\z\(sq\s36\(sq
        !          1104: .P2
        !          1105: is produced by
        !          1106: .P1 0
        !          1107: ^sp 2
        !          1108: \es8\ez\e(sq\es14\ez\e(sq\es22\ez\e(sq\es36\e(sq
        !          1109: .P2
        !          1110: The
        !          1111: .CW .sp
        !          1112: is needed to leave room for the result.
        !          1113: .PP
        !          1114: As another example, an extra-heavy semicolon
        !          1115: that looks like
        !          1116: .P1
        !          1117: .ft 1
        !          1118: \s+6\z,\v'-0.25m'.\v'0.25m'\s0  instead of  ;  or  \s+6;\s0
        !          1119: .P2
        !          1120: can be constructed with a big comma and a big period above it:
        !          1121: .P1
        !          1122: \es+6\ez,\ev'\(mi0.25m'.\ev'0.25m'\es0 
        !          1123: .P2
        !          1124: `0.25m' is an empirical constant.
        !          1125: .SP
        !          1126: .PP
        !          1127: A more ornate overstrike is given by the bracketing function
        !          1128: .CW \eb ,
        !          1129: which piles up characters vertically,
        !          1130: centered on the current baseline.
        !          1131: Thus we can get big brackets,
        !          1132: constructing them with piled-up smaller pieces:
        !          1133: .P1
        !          1134: .sp
        !          1135: .ne 3
        !          1136: \b'\(lt\(lk\(lb' \b'\(lc\(lf' x \b'\(rc\(rf' \b'\(rt\(rk\(rb'
        !          1137: .sp
        !          1138: .P2
        !          1139: by typing in only this:
        !          1140: .P1 0
        !          1141: .ps -2
        !          1142: \&^sp
        !          1143: \eb'\e(lt\e(lk\e(lb' \eb'\e(lc\e(lf' x \e
        !          1144: \eb'\e(rc\e(rf' \eb'\e(rt\e(rk\e(rb'
        !          1145: .P2
        !          1146: .PP
        !          1147: .IT Troff
        !          1148: also provides a convenient facility for drawing horizontal and vertical
        !          1149: lines of arbitrary length with arbitrary characters.
        !          1150: .CW \el'1i'
        !          1151: draws a line one inch long, like this:
        !          1152: \l'1i'\|.
        !          1153: The length can be followed by
        !          1154: the character to use if the \(ru isn't appropriate;
        !          1155: .CW \el'0.5i.'\&
        !          1156: draws a half-inch line of dots: \l'.5i.'.
        !          1157: The construction
        !          1158: .CW \eL
        !          1159: is analogous to
        !          1160: .CW \el ,
        !          1161: except that it draws a vertical line instead of horizontal.
        !          1162: .NH
        !          1163: Strings
        !          1164: .PP
        !          1165: Obviously if a paper contains a large number of occurrences
        !          1166: of an acute accent over a letter `e',
        !          1167: typing
        !          1168: .CW \eo"e\e\'"
        !          1169: for each \*e
        !          1170: would be a great nuisance.
        !          1171: .PP
        !          1172: Fortunately,
        !          1173: .IT troff
        !          1174: provides a way in which you can store an arbitrary
        !          1175: collection of text in a `string',
        !          1176: and thereafter use the string name as a shorthand
        !          1177: for its contents.
        !          1178: Strings are one of several
        !          1179: .IT troff
        !          1180: mechanisms whose judicious use
        !          1181: lets you type a document
        !          1182: with less effort and organize
        !          1183: it
        !          1184: so that extensive format changes
        !          1185: can be made with few editing changes.
        !          1186: .PP
        !          1187: Strings are defined with the command
        !          1188: .CW .ds .
        !          1189: The line
        !          1190: .P1
        !          1191: \&^ds e \eo"e\e'"
        !          1192: .P2
        !          1193: defines the string
        !          1194: .CW e
        !          1195: to have the value
        !          1196: .CW \eo"e\e\'" .
        !          1197: .PP
        !          1198: A reference to a string is replaced by whatever
        !          1199: text
        !          1200: the string was defined as.
        !          1201: String names may be either one or two characters long,
        !          1202: and are referred to by
        !          1203: .CW \e*x
        !          1204: for one character names or
        !          1205: .CW \e*(xy
        !          1206: for two character names.
        !          1207: Thus to get
        !          1208: .ds e \o"e\*'"
        !          1209: t\*el\*ephone,
        !          1210: given the definition of the string
        !          1211: .CW e
        !          1212: as above,
        !          1213: we can type
        !          1214: .CW t\e*el\e*ephone ,
        !          1215: to produce t\*el\*ephone.
        !          1216: .PP
        !          1217: If a string must begin with blanks, define it as
        !          1218: .P1
        !          1219: \&.ds xx "      text
        !          1220: .P2
        !          1221: The double quote signals the beginning of the definition.
        !          1222: There is no trailing quote;
        !          1223: the end of the line terminates the string.
        !          1224: .PP
        !          1225: A string may actually be several lines long;
        !          1226: if
        !          1227: .IT troff
        !          1228: encounters a 
        !          1229: .CW \e
        !          1230: at the end of
        !          1231: .ul
        !          1232: any
        !          1233: line, it is thrown away and the next line
        !          1234: added to the current one.
        !          1235: So you can make a long string simply by ending each line
        !          1236: but the last with a backslash:
        !          1237: .P1
        !          1238: \&^ds xx this \e
        !          1239: is a very \e
        !          1240: long string
        !          1241: .P2
        !          1242: .PP
        !          1243: Strings may be defined in terms of other strings, or even in terms of themselves;
        !          1244: we will discuss some of these possibilities later.
        !          1245: .NH
        !          1246: Introduction to Macros
        !          1247: .PP
        !          1248: Before we can go much further in
        !          1249: .IT troff ,
        !          1250: we need to learn a bit about the
        !          1251: macro
        !          1252: facility.
        !          1253: In its simplest form, a macro is just a shorthand notation
        !          1254: quite similar to a string.
        !          1255: Suppose we want every paragraph to start
        !          1256: in exactly the same way \(em
        !          1257: with a space and a temporary indent of two ems:
        !          1258: .P1
        !          1259: ^sp
        !          1260: ^ti +2m
        !          1261: .P2
        !          1262: Then to save typing, we would like to collapse these into
        !          1263: one shorthand line,
        !          1264: a
        !          1265: .IT troff
        !          1266: `command' like
        !          1267: .P1
        !          1268: ^PP
        !          1269: .P2
        !          1270: that would be treated by
        !          1271: .IT troff
        !          1272: exactly as
        !          1273: .P1
        !          1274: ^sp
        !          1275: ^ti +2m
        !          1276: .P2
        !          1277: .CW .PP
        !          1278: is called a
        !          1279: .ul
        !          1280: macro.
        !          1281: The way we tell
        !          1282: .IT troff
        !          1283: what
        !          1284: .CW .PP
        !          1285: means is to
        !          1286: .ul
        !          1287: define
        !          1288: it with the
        !          1289: .CW .de
        !          1290: command:
        !          1291: .P1
        !          1292: ^de PP
        !          1293: ^sp
        !          1294: ^ti +2m
        !          1295: ^^
        !          1296: .P2
        !          1297: The first line names the macro
        !          1298: (we used
        !          1299: .CW .PP ' `
        !          1300: for `paragraph',
        !          1301: and upper case so it wouldn't conflict with
        !          1302: any name that
        !          1303: .IT troff
        !          1304: might
        !          1305: already know about).
        !          1306: The last line
        !          1307: .CW ..\&
        !          1308: marks the end of the definition.
        !          1309: In between is the text,
        !          1310: which is simply inserted whenever
        !          1311: .IT troff
        !          1312: sees the `command'
        !          1313: or macro call
        !          1314: .P1
        !          1315: ^PP
        !          1316: .P2
        !          1317: A macro
        !          1318: can contain any mixture of text and formatting commands.
        !          1319: .PP
        !          1320: The definition of
        !          1321: .CW .PP
        !          1322: has to precede its first use;
        !          1323: undefined macros are simply ignored.
        !          1324: Names are restricted to one or two characters.
        !          1325: .PP
        !          1326: Using macros for commonly occurring sequences of commands
        !          1327: is critically important.
        !          1328: Not only does it save typing,
        !          1329: but it makes later changes much easier.
        !          1330: Suppose we decide that the paragraph indent is too small,
        !          1331: the vertical space is much too big,
        !          1332: and roman font should be forced.
        !          1333: Instead of changing the whole document,
        !          1334: we need only change the definition of
        !          1335: .CW .PP
        !          1336: to
        !          1337: something like
        !          1338: .P1
        !          1339: ^de PP \e" paragraph macro
        !          1340: ^sp 2p
        !          1341: ^ti +3m
        !          1342: ^ft R
        !          1343: ^^
        !          1344: .P2
        !          1345: and the change takes
        !          1346: effect everywhere we used
        !          1347: .CW .PP .
        !          1348: .PP
        !          1349: .CW \e"
        !          1350: is a
        !          1351: .IT troff
        !          1352: command that causes the rest of the line to be ignored.
        !          1353: We use it here to add comments to the macro
        !          1354: definition
        !          1355: (a wise idea once definitions get complicated).
        !          1356: .PP
        !          1357: As another example of macros,
        !          1358: consider these two which start and end a block of offset,
        !          1359: unfilled text, like most of the examples in this paper:
        !          1360: .P1
        !          1361: ^de BS \e" start indented block
        !          1362: ^sp
        !          1363: ^nf
        !          1364: ^in +0.3i
        !          1365: ^ft CW  \e" constant width font
        !          1366: ^^
        !          1367: ^de BE \e" end indented block
        !          1368: ^sp
        !          1369: ^ft 1
        !          1370: ^fi
        !          1371: ^in \(mi0.3i
        !          1372: ^^
        !          1373: .P2
        !          1374: Now we can surround text like
        !          1375: .P1
        !          1376: Copy to
        !          1377: John Doe
        !          1378: Richard Roberts
        !          1379: Stanley Smith
        !          1380: .P2
        !          1381: by the commands
        !          1382: .CW .BS
        !          1383: and
        !          1384: .CW .BE ,
        !          1385: and it will come out as it did above.
        !          1386: Notice that we indented by
        !          1387: .CW .in
        !          1388: .CW +0.3i
        !          1389: instead of
        !          1390: .CW .in
        !          1391: .CW 0.3i .
        !          1392: This way we can nest our uses of
        !          1393: .CW .BS
        !          1394: and
        !          1395: .CW .BE
        !          1396: to get blocks within blocks.
        !          1397: .PP
        !          1398: If later on we decide that the indent
        !          1399: should be 0.5i, then it is only necessary to
        !          1400: change the definitions of
        !          1401: .CW .BS
        !          1402: and
        !          1403: .CW .BE ,
        !          1404: not the whole paper.
        !          1405: .NH
        !          1406: Titles, Pages and Numbering
        !          1407: .PP
        !          1408: This is an area where things get tougher,
        !          1409: because nothing is done for you automatically.
        !          1410: Of necessity, some of this section is a cookbook,
        !          1411: to be copied literally until you get some experience.
        !          1412: .PP
        !          1413: Suppose you want a title at the top of each page,
        !          1414: saying just
        !          1415: .sp 3p
        !          1416: .lt 2.8i
        !          1417: .tl '\ \ \ \ left top'center top'right top\ \ \ \ '
        !          1418: .lt
        !          1419: .sp 3p
        !          1420: Unfortunately, this requires work.
        !          1421: .PP
        !          1422: You have to say what the actual title is (easy);
        !          1423: when to print it (easy enough);
        !          1424: and what to do at and around the title line (harder).
        !          1425: Taking these in reverse order,
        !          1426: first we define a macro
        !          1427: .CW .NP
        !          1428: (for `new page') to process
        !          1429: titles and the like at the end of one page
        !          1430: and the beginning of the next:
        !          1431: .P1 .2i
        !          1432: ^de NP
        !          1433: \&'bp
        !          1434: \&'sp 0.5i
        !          1435: \&\&.tl 'left top'center top'right top'
        !          1436: \&'sp 0.3i
        !          1437: \&^^
        !          1438: .P2
        !          1439: To make sure we're at the top of a page,
        !          1440: we issue a `begin page' command
        !          1441: .CW 'bp ,
        !          1442: which causes a skip to top-of-page
        !          1443: (we'll explain the
        !          1444: .CW '
        !          1445: shortly).
        !          1446: Then we space down half an inch,
        !          1447: print the title
        !          1448: (the use of
        !          1449: .CW .tl
        !          1450: should be self explanatory; later we will discuss parameterizing the titles),
        !          1451: space another 0.3 inches,
        !          1452: and we're done.
        !          1453: .PP
        !          1454: To ask for
        !          1455: .CW .NP
        !          1456: at the bottom of each page,
        !          1457: we have to say something like
        !          1458: `when the text is within an inch
        !          1459: of the bottom of the page,
        !          1460: start the processing
        !          1461: for a new page.'
        !          1462: This is done with a `when' command
        !          1463: .CW .wh :
        !          1464: .P1
        !          1465: ^wh \-1i NP
        !          1466: .P2
        !          1467: (No `.' is used before
        !          1468: .CW NP ;
        !          1469: this is simply the name of a macro, not a macro call.)
        !          1470: The minus sign means
        !          1471: `measure up from the bottom of the page',
        !          1472: so
        !          1473: .CW -1i
        !          1474: means `one inch from the bottom'.
        !          1475: .PP
        !          1476: The
        !          1477: .CW .wh
        !          1478: command appears in the input outside the definition of
        !          1479: .CW .NP ;
        !          1480: typically the input would be
        !          1481: .P1
        !          1482: ^de NP
        !          1483: ^^^
        !          1484: ^^
        !          1485: ^wh \-1i NP
        !          1486: .P2
        !          1487: .PP
        !          1488: Now what happens?
        !          1489: As text is actually being output,
        !          1490: .IT troff 
        !          1491: keeps track of its vertical position on the page,
        !          1492: and after a line is printed within one inch from the bottom,
        !          1493: the
        !          1494: .CW .NP
        !          1495: macro is activated.
        !          1496: (In the jargon, the
        !          1497: .CW .wh
        !          1498: command sets a
        !          1499: .ul
        !          1500: trap
        !          1501: at the specified place,
        !          1502: which is `sprung' when that point is passed.)
        !          1503: .CW .NP
        !          1504: causes a skip to the top of the next page
        !          1505: (that's what the
        !          1506: .CW 'bp
        !          1507: was for),
        !          1508: then prints the title with the appropriate margins.
        !          1509: .PP
        !          1510: Why
        !          1511: .CW 'bp
        !          1512: and
        !          1513: .CW 'sp 
        !          1514: instead of
        !          1515: .CW .bp
        !          1516: and
        !          1517: .CW .sp ?
        !          1518: The answer is that
        !          1519: .CW .sp
        !          1520: and
        !          1521: .CW .bp ,
        !          1522: like several other commands,
        !          1523: cause a
        !          1524: .ul
        !          1525: break
        !          1526: to take place.
        !          1527: That is, all the input text collected but not yet printed
        !          1528: is flushed out as soon as possible,
        !          1529: and the next input line is guaranteed to start
        !          1530: a new line of output.
        !          1531: If we had used
        !          1532: .CW .sp
        !          1533: or
        !          1534: .CW .bp
        !          1535: in the
        !          1536: .CW .NP
        !          1537: macro,
        !          1538: this would cause a break in the middle
        !          1539: of the current output line when a new page is started.
        !          1540: The effect would be to print the left-over part of that line
        !          1541: at the top of the page, followed by the next input line on a new output line.
        !          1542: This is
        !          1543: .ul
        !          1544: not
        !          1545: what we want.
        !          1546: Using
        !          1547: .CW '
        !          1548: instead of
        !          1549: .CW . \&
        !          1550: for a command
        !          1551: tells
        !          1552: .IT troff 
        !          1553: that
        !          1554: no break is to take place \(em
        !          1555: the output line
        !          1556: currently being filled
        !          1557: should
        !          1558: .ul
        !          1559: not
        !          1560: be forced out before the space or new page.
        !          1561: .PP
        !          1562: The list of commands that cause a break 
        !          1563: is short and natural:
        !          1564: .P1 .1i
        !          1565: ^bp  ^br  ^ce  ^fi  ^nf  ^sp  ^in  ^ti
        !          1566: .P2
        !          1567: All others cause
        !          1568: .ul
        !          1569: no
        !          1570: break,
        !          1571: regardless of whether you use a
        !          1572: .CW . \&
        !          1573: or a 
        !          1574: .CW ' .
        !          1575: If you really need a break, add a
        !          1576: .CW .br 
        !          1577: command at the appropriate place.
        !          1578: .PP
        !          1579: One other thing to beware of \(em
        !          1580: if you're changing fonts or point sizes a lot,
        !          1581: you may find that
        !          1582: if you cross a page boundary
        !          1583: in an unexpected font or size,
        !          1584: your titles come out in that size and font
        !          1585: instead of what you intended.
        !          1586: Furthermore, the length of a title is independent of the current line length,
        !          1587: so titles will come out at the default length of 6.5 inches
        !          1588: unless you change it,
        !          1589: which is done with the
        !          1590: .CW .lt
        !          1591: command.
        !          1592: .PP
        !          1593: There are several ways to fix the problems of point sizes
        !          1594: and fonts in titles.
        !          1595: For the simplest applications, we can change
        !          1596: .CW .NP 
        !          1597: to set the proper size and font for the title,
        !          1598: then restore the previous values as shown below.
        !          1599: .KF
        !          1600: .P1 .2i
        !          1601: .ta .8i
        !          1602: ^de NP
        !          1603: \&'bp
        !          1604: \&'sp 0.5i
        !          1605: ^ft R  \e" set title font to roman
        !          1606: ^ps 10 \e" and size to 10 point
        !          1607: ^lt 6i \e" and length to 6 inches
        !          1608: ^tl 'left'center'right'
        !          1609: ^ps    \e" revert to previous size
        !          1610: ^ft P  \e" and to previous font
        !          1611: \&'sp 0.3i
        !          1612: ^^
        !          1613: .P2
        !          1614: .KE
        !          1615: .PP
        !          1616: This version of
        !          1617: .CW .NP
        !          1618: does
        !          1619: .ul
        !          1620: not
        !          1621: work if the fields in the
        !          1622: .CW .tl
        !          1623: command contain size or font changes.
        !          1624: To cope with that
        !          1625: requires
        !          1626: .IT troff 's
        !          1627: `environment' mechanism,
        !          1628: which we will discuss in Section 13.
        !          1629: .PP
        !          1630: To get a footer at the bottom of a page,
        !          1631: you can modify
        !          1632: .CW .NP
        !          1633: so it does
        !          1634: some processing before
        !          1635: the
        !          1636: .CW 'bp
        !          1637: command,
        !          1638: or split the job into a footer macro invoked
        !          1639: at the bottom margin and a header macro invoked
        !          1640: at the top of the page.
        !          1641: These variations are left as exercises.
        !          1642: .SP
        !          1643: .PP
        !          1644: Output page numbers are computed automatically
        !          1645: as each page is produced (starting at 1),
        !          1646: but no numbers are printed unless you ask for them explicitly.
        !          1647: To get page numbers printed,
        !          1648: include the character
        !          1649: .CW %
        !          1650: in the
        !          1651: .CW .tl
        !          1652: line at
        !          1653: the position where you want the number to appear.
        !          1654: For example
        !          1655: .P1
        !          1656: ^tl ''- % -''
        !          1657: .P2
        !          1658: centers the page number inside hyphens, as on this page.
        !          1659: You can set the page number at any time
        !          1660: with either
        !          1661: .CW .bp\ n ,
        !          1662: which immediately starts a new page numbered
        !          1663: .CW n ,
        !          1664: or with
        !          1665: .CW .pn\ n ,
        !          1666: which sets the page number for the next page
        !          1667: but doesn't cause a skip to the new page.
        !          1668: Again,
        !          1669: .CW .bp\ +n
        !          1670: sets the page number to
        !          1671: .CW n
        !          1672: more than its current value;
        !          1673: .CW .bp
        !          1674: means
        !          1675: .CW .bp\ +1 .
        !          1676: .NH
        !          1677: Number Registers and Arithmetic
        !          1678: .PP
        !          1679: .IT Troff
        !          1680: has a facility for doing arithmetic,
        !          1681: and for defining and using variables with numeric values,
        !          1682: called
        !          1683: .ul
        !          1684: number registers.
        !          1685: Number registers, like strings and macros, can be useful in setting up a document
        !          1686: so it is easy to change later.
        !          1687: And of course they serve for any sort of arithmetic computation.
        !          1688: .PP
        !          1689: Like strings, number registers have one or two character names.
        !          1690: They are set by the
        !          1691: .CW .nr
        !          1692: command,
        !          1693: and are referenced anywhere by
        !          1694: .CW \enx
        !          1695: (one character name) or
        !          1696: .CW \en(xy
        !          1697: (two character name).
        !          1698: .PP
        !          1699: There are quite a few pre-defined number registers maintained by
        !          1700: .IT troff ,
        !          1701: among them
        !          1702: .CW %
        !          1703: for the current page number;
        !          1704: .CW nl
        !          1705: for the current vertical position on the page;
        !          1706: .CW dy ,
        !          1707: .CW mo
        !          1708: and
        !          1709: .CW yr
        !          1710: for the current day, month and year; and
        !          1711: .CW .s
        !          1712: and
        !          1713: .CW .f
        !          1714: for the current size and font.
        !          1715: (The font is a number from 1 to typically around 10.)
        !          1716: Any of these can be used in computations like any other register,
        !          1717: but some, like
        !          1718: .CW .s
        !          1719: and
        !          1720: .CW .f ,
        !          1721: cannot be changed with
        !          1722: .CW .nr .
        !          1723: .PP
        !          1724: As an example of the use of number registers,
        !          1725: in the
        !          1726: .CW -ms
        !          1727: macro package,
        !          1728: most significant parameters are defined in terms of the values
        !          1729: of a handful of number registers.
        !          1730: These include the point size for text, the vertical spacing,
        !          1731: and the line and title lengths.
        !          1732: To set the point size and vertical spacing for the following paragraphs, for example, a user may say
        !          1733: .P1
        !          1734: ^nr PS 9
        !          1735: ^nr VS 11
        !          1736: .P2
        !          1737: The paragraph macro
        !          1738: .CW .PP
        !          1739: is defined (roughly) as follows:
        !          1740: .P1
        !          1741: .ta  1i
        !          1742: ^de PP
        !          1743: ^ps \e\en(PS   \e" reset size
        !          1744: ^vs \e\en(VSp  \e" spacing
        !          1745: ^ft 1  \e" font
        !          1746: ^sp 0.5v       \e" half a line
        !          1747: ^ti +3m
        !          1748: ^^
        !          1749: .P2
        !          1750: This sets the font to font 1 and the point size and line spacing
        !          1751: to whatever values are stored in the number registers
        !          1752: .CW PS
        !          1753: and
        !          1754: .CW VS .
        !          1755: .PP
        !          1756: Why are there two backslashes?
        !          1757: This is the eternal problem of how to quote a quote.
        !          1758: When
        !          1759: .IT troff
        !          1760: originally reads the macro definition,
        !          1761: it peels off one backslash
        !          1762: to see what's coming next.
        !          1763: To ensure that another is left in the definition when the 
        !          1764: macro is
        !          1765: .ul
        !          1766: used,
        !          1767: we have to put in two backslashes in the definition.
        !          1768: If only one backslash is used, 
        !          1769: point size and vertical spacing will be frozen at the time the macro
        !          1770: is defined, not when it is used.
        !          1771: .PP
        !          1772: Protecting by an extra layer of backslashes
        !          1773: is only needed for
        !          1774: .CW \en ,
        !          1775: .CW \e* ,
        !          1776: .CW \e$
        !          1777: (which we haven't come to yet),
        !          1778: and
        !          1779: .CW \e
        !          1780: itself.
        !          1781: Things like
        !          1782: .CW \es ,
        !          1783: .CW \ef ,
        !          1784: .CW \eh ,
        !          1785: .CW \ev ,
        !          1786: and so on do not need an extra backslash,
        !          1787: since they are converted by
        !          1788: .IT troff
        !          1789: to an internal code immediately upon being seen.
        !          1790: .SP
        !          1791: .PP
        !          1792: Arithmetic expressions can appear anywhere that
        !          1793: a number is expected.
        !          1794: As a trivial example,
        !          1795: .P1
        !          1796: ^nr PS \e\en(PS\-2
        !          1797: .P2
        !          1798: decrements
        !          1799: .CW PS
        !          1800: by 2.
        !          1801: Expressions can use the arithmetic operators +, \-, *, /, % (mod),
        !          1802: the relational operators >, >=, <, <=, =, and != (not equal),
        !          1803: and parentheses.
        !          1804: .PP
        !          1805: Although the arithmetic we have done so far
        !          1806: has been straightforward,
        !          1807: more complicated things are somewhat tricky.
        !          1808: First,
        !          1809: number registers hold only integers.
        !          1810: .IT troff
        !          1811: arithmetic uses truncating integer division, just like Fortran.
        !          1812: Second, in the absence of parentheses,
        !          1813: evaluation is done left-to-right
        !          1814: without any operator precedence
        !          1815: (including relational operators).
        !          1816: Thus
        !          1817: .P1
        !          1818: 7*\-4+3/13
        !          1819: .P2
        !          1820: becomes `\-1'.
        !          1821: Number registers can occur anywhere in an expression,
        !          1822: and so can scale indicators like
        !          1823: .CW p ,
        !          1824: .CW i ,
        !          1825: .CW m ,
        !          1826: and so on (but no spaces).
        !          1827: Although integer division causes truncation,
        !          1828: each number and its scale indicator is converted
        !          1829: to machine units before any arithmetic is done,
        !          1830: so
        !          1831: .CW 1i/2u
        !          1832: evaluates to
        !          1833: 0.5i
        !          1834: correctly.
        !          1835: .PP
        !          1836: The scale indicator
        !          1837: .CW u
        !          1838: often has to appear
        !          1839: when you wouldn't expect it \(em
        !          1840: in particular, when arithmetic is being done
        !          1841: in a context that implies horizontal or vertical dimensions.
        !          1842: For example,
        !          1843: .P1
        !          1844: ^ll 7/2i
        !          1845: .P2
        !          1846: would seem obvious enough \(em
        !          1847: 3\(12 inches.
        !          1848: Sorry.
        !          1849: Remember that the default units for horizontal parameters like
        !          1850: .CW .ll
        !          1851: are ems.
        !          1852: That's really `7 ems / 2 inches',
        !          1853: and when translated into machine units, it becomes zero.
        !          1854: How about
        !          1855: .P1
        !          1856: ^ll 7i/2
        !          1857: .P2
        !          1858: Sorry, still no good \(em
        !          1859: the `2' is `2 ems', so `7i/2' is small,
        !          1860: although not zero.
        !          1861: You
        !          1862: .ul
        !          1863: must
        !          1864: use
        !          1865: .P1
        !          1866: ^ll 7i/2u
        !          1867: .P2
        !          1868: So again, a safe rule is to
        !          1869: attach a scale indicator to every number,
        !          1870: even constants.
        !          1871: .PP
        !          1872: For arithmetic done within a
        !          1873: .CW .nr
        !          1874: command,
        !          1875: there is no implication of horizontal or vertical dimension,
        !          1876: so the default units are `units',
        !          1877: and
        !          1878: .CW 7i/2
        !          1879: and
        !          1880: .CW 7i/2u
        !          1881: mean the same thing.
        !          1882: Thus
        !          1883: .P1
        !          1884: ^nr ll 7i/2
        !          1885: ^ll \e\en(llu
        !          1886: .P2
        !          1887: does just what you want,
        !          1888: so long as you
        !          1889: don't forget the
        !          1890: .CW u
        !          1891: on the
        !          1892: .CW .ll
        !          1893: command.
        !          1894: .NH
        !          1895: Macros with arguments
        !          1896: .PP
        !          1897: The next step is to define macros that can change from one
        !          1898: use to the next
        !          1899: according to parameters supplied as arguments.
        !          1900: To make this work, we need two things.
        !          1901: First, when we define the macro, we have to indicate that some
        !          1902: parts of it will be provided as arguments when the macro is called.
        !          1903: Then when the macro is 
        !          1904: called
        !          1905: we have to provide actual arguments
        !          1906: to be plugged into the definition.
        !          1907: .PP
        !          1908: Let us illustrate by defining a macro
        !          1909: .CW .SM
        !          1910: that will print its argument two points
        !          1911: smaller than the surrounding text.
        !          1912: That is, the macro call
        !          1913: .P1
        !          1914: ^SM UNIX
        !          1915: .P2
        !          1916: will produce
        !          1917: \s-2UNIX\s0
        !          1918: instead of UNIX.
        !          1919: .PP
        !          1920: The definition of
        !          1921: .CW .SM
        !          1922: is
        !          1923: .P1
        !          1924: ^de SM
        !          1925: \es\-2\e\e$1\es+2
        !          1926: ^^
        !          1927: .P2
        !          1928: Within a macro definition,
        !          1929: the symbol
        !          1930: .CW \e\e$n
        !          1931: refers to the
        !          1932: .CW n th
        !          1933: argument
        !          1934: that the macro was called with.
        !          1935: Thus
        !          1936: .CW \e\e$1
        !          1937: is the string to be placed in a smaller point
        !          1938: size when
        !          1939: .CW .SM
        !          1940: is called.
        !          1941: .PP
        !          1942: As a slightly more complicated version, the following definition of
        !          1943: .CW .SM
        !          1944: permits optional second and third arguments
        !          1945: that will be printed in the normal size:
        !          1946: .P1
        !          1947: ^de SM
        !          1948: \e\e$3\es\-2\e\e$1\es+2\e\e$2
        !          1949: ^^
        !          1950: .P2
        !          1951: Arguments not provided when the macro is called are treated
        !          1952: as empty,
        !          1953: so
        !          1954: .P1
        !          1955: ^SM UNIX ),
        !          1956: .P2
        !          1957: produces
        !          1958: .UC UNIX ),
        !          1959: while
        !          1960: .P1
        !          1961: ^SM UNIX ). (
        !          1962: .P2
        !          1963: produces
        !          1964: .UC UNIX ). (
        !          1965: It is convenient to reverse 
        !          1966: the order of arguments because trailing punctuation
        !          1967: is much more common than leading.
        !          1968: .PP
        !          1969: By the way, the number of arguments that a macro was called with
        !          1970: is available in number register
        !          1971: .CW .$ .
        !          1972: .PP
        !          1973: The following macro
        !          1974: .CW ^BD
        !          1975: is used to make
        !          1976: `bold roman',
        !          1977: .BD like
        !          1978: .BD this
        !          1979: .BD phrase.
        !          1980: It combines horizontal motions, width computations,
        !          1981: and argument rearrangement.
        !          1982: .P1 .05i
        !          1983: \&.de BD
        !          1984: \e&\e\e$3\ef1\e\e$1\eh'-\ew'\e\e$1'u+3u'\e\e$1\efP\e\e$2
        !          1985: \&..
        !          1986: .P2
        !          1987: The
        !          1988: .CW \eh
        !          1989: and
        !          1990: .CW \ew
        !          1991: commands need no extra backslash, as we discussed above.
        !          1992: The
        !          1993: .CW \e&
        !          1994: is there in case the argument begins with a period.
        !          1995: .SP
        !          1996: .PP
        !          1997: Two backslashes are needed with the
        !          1998: .CW \e\e$n
        !          1999: commands, though,
        !          2000: to protect one of them when the macro is
        !          2001: being defined.
        !          2002: Perhaps a second example will make this clearer.
        !          2003: Consider a macro called
        !          2004: .CW .NH
        !          2005: which
        !          2006: produces numbered headings rather like those in this paper,
        !          2007: with the sections numbered automatically,
        !          2008: and the title in bold in a smaller size.
        !          2009: The use is
        !          2010: .P1
        !          2011: ^NH "Section title ..."
        !          2012: .P2
        !          2013: (If the argument to a macro is to contain blanks,
        !          2014: then it must be
        !          2015: .ul
        !          2016: surrounded
        !          2017: by double quotes,
        !          2018: unlike a string, where only one leading quote is permitted.)
        !          2019: .PP
        !          2020: Here is the definition of the
        !          2021: .CW .NH
        !          2022: macro:
        !          2023: .P1 .1i
        !          2024: .ta .75i 1.15i
        !          2025: ^nr NH 0  \e" initialize section number
        !          2026: ^de NH
        !          2027: ^sp 0.3i
        !          2028: ^ft B
        !          2029: ^nr NH \e\en(NH+1      \e" increment number
        !          2030: ^ps \e\en(PS\-1        \e" decrease PS
        !          2031: \e\en(NH.  \e\e$1      \e" number. title
        !          2032: ^ps \e\en(PS           \e" restore PS
        !          2033: ^sp 0.3i
        !          2034: ^ft R
        !          2035: ^^
        !          2036: .P2
        !          2037: The section number is kept in number register
        !          2038: .CW NH ,
        !          2039: which is incremented each
        !          2040: time just before it is used.
        !          2041: (A number register, but not a string, may have the same name as a macro
        !          2042: without conflict.)
        !          2043: .PP
        !          2044: We used
        !          2045: .CW \e\en(NH
        !          2046: instead of
        !          2047: .CW \en(NH
        !          2048: and
        !          2049: .CW \e\en(PS
        !          2050: instead of
        !          2051: .CW \en(PS .
        !          2052: If we had used
        !          2053: .CW \en(NH ,
        !          2054: we would get the value of the register at the time the macro was
        !          2055: .ul
        !          2056: defined,
        !          2057: not at the time it was
        !          2058: .ul
        !          2059: used.
        !          2060: If that's what you want, fine,
        !          2061: but not here.
        !          2062: Similarly,
        !          2063: by using
        !          2064: .CW \e\en(PS ,
        !          2065: we get the point size at the time the macro is called.
        !          2066: .SP
        !          2067: .PP
        !          2068: As an example that does not involve numbers,
        !          2069: recall our
        !          2070: .CW .NP
        !          2071: macro which had a
        !          2072: .P1
        !          2073: ^tl 'left'center'right'
        !          2074: .P2
        !          2075: We could make these into parameters by using instead
        !          2076: .P1
        !          2077: ^tl '\e\e*(LT'\e\e*(CT'\e\e*(RT'
        !          2078: .P2
        !          2079: so the title comes from three strings called
        !          2080: .CW LT ,
        !          2081: .CW CT
        !          2082: and 
        !          2083: .CW RT .
        !          2084: If these are empty, then the title will be a blank line.
        !          2085: Normally
        !          2086: .CW CT
        !          2087: would be set with something like
        !          2088: .P1
        !          2089: \&^ds CT - % -
        !          2090: .P2
        !          2091: to give just the page number between hyphens,
        !          2092: but a user could supply private definitions for
        !          2093: any of the strings.
        !          2094: .NH
        !          2095: Conditionals
        !          2096: .PP
        !          2097: Suppose we want the
        !          2098: .CW .NH
        !          2099: macro to leave two extra inches of space just before section 1,
        !          2100: but nowhere else.
        !          2101: The cleanest way to do that is to test inside the
        !          2102: .CW .NH
        !          2103: macro
        !          2104: whether
        !          2105: the section number is 1,
        !          2106: and add some space if it is.
        !          2107: The
        !          2108: .CW .if
        !          2109: command provides the conditional test
        !          2110: that we can add
        !          2111: just before the heading line is output:
        !          2112: .P1 0
        !          2113:  ^if \e\en(NH=1 ^sp 2i \e" 1st section only
        !          2114: .P2
        !          2115: .PP
        !          2116: The condition after the
        !          2117: .CW .if
        !          2118: can be any arithmetic or logical expression.
        !          2119: If the condition is logically true, or arithmetically greater than zero,
        !          2120: the rest of the line is treated as if
        !          2121: it were text \(em
        !          2122: here a command.
        !          2123: If the condition is false, or zero or negative,
        !          2124: the rest of the line is skipped.
        !          2125: .PP
        !          2126: It is possible to do more than one command if a condition is true.
        !          2127: Suppose several operations are to be done before section 1.
        !          2128: One possibility is to define a macro 
        !          2129: .CW .S1
        !          2130: and invoke it
        !          2131: if we are about to do section 1
        !          2132: (as determined by an
        !          2133: .CW .if ).
        !          2134: .P1
        !          2135: ^de S1
        !          2136: ---  processing for section 1 ---
        !          2137: ^^
        !          2138: ^de NH
        !          2139: ^^^
        !          2140: ^if \e\en(NH=1 ^S1
        !          2141: ^^^
        !          2142: ^^
        !          2143: .P2
        !          2144: .PP
        !          2145: An alternate way is to use the
        !          2146: extended form of the
        !          2147: .CW .if ,
        !          2148: like this:
        !          2149: .P1
        !          2150: ^if \e\en(NH=1 \e{--- processing
        !          2151: for section 1 ----\e}
        !          2152: .P2
        !          2153: The braces
        !          2154: .CW \e{
        !          2155: and
        !          2156: .CW \e}
        !          2157: must occur in the positions shown
        !          2158: or you will get unexpected extra lines in your output.
        !          2159: .IT Troff
        !          2160: also provides
        !          2161: an `if-else' construction,
        !          2162: which we will not go into here.
        !          2163: .PP
        !          2164: A condition can be negated by preceding it with
        !          2165: .CW ! ;
        !          2166: we get the same effect as above (but less clearly) by using
        !          2167: .P1
        !          2168: ^if !\e\en(NH>1 ^S1
        !          2169: .P2
        !          2170: .PP
        !          2171: There are a handful of 
        !          2172: other conditions that can be tested with
        !          2173: .CW .if .
        !          2174: For example, is the current page even or odd?
        !          2175: .P1
        !          2176: ^if e ^tl ''even page title''
        !          2177: ^if o ^tl ''odd page title''
        !          2178: .P2
        !          2179: gives facing pages different titles
        !          2180: when used inside an appropriate new page macro.
        !          2181: .PP
        !          2182: Two other conditions
        !          2183: are
        !          2184: .CW t
        !          2185: and
        !          2186: .CW n ,
        !          2187: which tell you whether the formatter is
        !          2188: .IT troff
        !          2189: or
        !          2190: .IT nroff . 
        !          2191: .P1
        !          2192: ^if t troff stuff ...
        !          2193: ^if n nroff stuff ...
        !          2194: .P2
        !          2195: .PP
        !          2196: Finally, string comparisons may be made in an
        !          2197: .CW .if :
        !          2198: .P1
        !          2199: ^if 'string1'string2' stuff
        !          2200: .P2
        !          2201: does `stuff' if
        !          2202: .CW string1
        !          2203: is the same as
        !          2204: .CW string2 .
        !          2205: The character separating the strings can be anything
        !          2206: reasonable that is
        !          2207: not contained in either string.
        !          2208: The strings themselves can reference strings with
        !          2209: .CW \e* ,
        !          2210: arguments with 
        !          2211: .CW \e$ ,
        !          2212: and so on.
        !          2213: .NH
        !          2214: Environments
        !          2215: .PP
        !          2216: As we mentioned, there is a potential problem
        !          2217: when going across a page boundary:
        !          2218: parameters like size and font
        !          2219: for a page title may well be different from those
        !          2220: in effect in the text when the page boundary occurs.
        !          2221: .IT Troff
        !          2222: provides a very general way to deal with this and
        !          2223: similar situations.
        !          2224: There are three `environments',
        !          2225: each of which has independently settable versions of
        !          2226: many of the parameters associated with processing,
        !          2227: including size, font, line and title lengths,
        !          2228: fill/nofill mode, tab stops, and even partially collected lines.
        !          2229: Thus the titling problem may be readily solved by processing the main text
        !          2230: in one environment and titles in a separate one
        !          2231: with its own suitable parameters.
        !          2232: .PP
        !          2233: The command
        !          2234: .CW .ev\ n
        !          2235: shifts to environment
        !          2236: .CW n ;
        !          2237: .CW n
        !          2238: must be 0, 1 or 2.
        !          2239: The command
        !          2240: .CW .ev
        !          2241: with no argument returns to the
        !          2242: previous environment.
        !          2243: Environment names are maintained in a stack, so calls
        !          2244: for different environments may be nested and unwound consistently.
        !          2245: .PP
        !          2246: Suppose we say that the main text is processed in environment 0,
        !          2247: which is where 
        !          2248: .IT troff
        !          2249: begins by default.
        !          2250: Then we can modify the new page macro
        !          2251: .CW .NP
        !          2252: to process titles in environment 1 like this:
        !          2253: .P1 2
        !          2254: ^de NP
        !          2255: ^ev 1  \e" shift to new environment
        !          2256: ^lt 6i \e" set parameters here
        !          2257: ^ft 1
        !          2258: ^ps 10
        !          2259: \&... any other processing ...
        !          2260: ^ev    \e" return to prev environment
        !          2261: ^^
        !          2262: .P2
        !          2263: It is also possible to initialize the parameters for an environment
        !          2264: outside the
        !          2265: .CW .NP
        !          2266: macro,
        !          2267: but the version shown keeps all the processing in one place
        !          2268: and is thus easier to understand and change.
        !          2269: .NH
        !          2270: Diversions
        !          2271: .PP
        !          2272: There are numerous occasions in page layout when it is necessary to store some text
        !          2273: for a period of time without actually printing it.
        !          2274: Footnotes are one obvious example:
        !          2275: the text of the footnote usually appears in the input well before the place
        !          2276: on the page where it is to be printed is reached.
        !          2277: In fact,
        !          2278: the place where it is output normally depends on how big it is,
        !          2279: which implies that there must be a way
        !          2280: to process the footnote at least
        !          2281: enough to decide its size
        !          2282: without printing it.
        !          2283: .PP
        !          2284: .IT Troff
        !          2285: provides a mechanism called a diversion
        !          2286: for doing this processing.
        !          2287: Any part of the output may be diverted into a macro instead
        !          2288: of being printed,
        !          2289: and then at some convenient time the macro may be put back into
        !          2290: the input.
        !          2291: .PP
        !          2292: The command
        !          2293: .CW .di\ xy
        !          2294: begins a diversion \(em all subsequent output is collected into the macro
        !          2295: .CW xy
        !          2296: until the command
        !          2297: .CW .di
        !          2298: with no arguments is encountered.
        !          2299: This terminates the diversion.
        !          2300: The processed text is available at any time thereafter, simply
        !          2301: by giving the command
        !          2302: .P1
        !          2303: ^xy
        !          2304: .P2
        !          2305: The vertical size of the last finished diversion is contained in
        !          2306: the built-in number register
        !          2307: .CW dn .
        !          2308: .PP
        !          2309: As a simple example,
        !          2310: suppose we want to implement a `keep-release'
        !          2311: operation,
        !          2312: so that text between the commands
        !          2313: .CW .KS 
        !          2314: and
        !          2315: .CW .KE
        !          2316: will not be split across a page boundary
        !          2317: (as for a figure or table).
        !          2318: Clearly, when a
        !          2319: .CW .KS
        !          2320: is encountered, we have to begin diverting
        !          2321: the output so we can find out how big it is.
        !          2322: Then when a
        !          2323: .CW .KE
        !          2324: is seen, we decide
        !          2325: whether the diverted text will fit on the current page,
        !          2326: and print it either there if it fits, or at the top of the next page if it doesn't.
        !          2327: So:
        !          2328: .P1 2
        !          2329: .ta .6i
        !          2330: ^de KS \e" start keep
        !          2331: ^br    \e" start fresh line
        !          2332: ^ev 1  \e" collect in new environment
        !          2333: ^fi    \e" make it filled text
        !          2334: ^di XX \e" collect in XX
        !          2335: ^^
        !          2336: .P2
        !          2337: .P1 2
        !          2338: .ta .6i
        !          2339: ^de KE \e" end keep
        !          2340: ^br    \e" get last partial line
        !          2341: ^di    \e" end diversion
        !          2342: ^if \e\en(dn>=\e\en(.t .bp \e" bp if too big
        !          2343: ^nf    \e" bring it back in no-fill
        !          2344: ^XX    \e" text
        !          2345: ^ev    \e" return to normal environment
        !          2346: ^^
        !          2347: .P2
        !          2348: Recall that number register
        !          2349: .CW nl
        !          2350: is the current position
        !          2351: on the output page.
        !          2352: Since output was being diverted, this remains
        !          2353: at its value when the diversion started.
        !          2354: .CW dn
        !          2355: is the amount of text in the diversion;
        !          2356: .CW .t
        !          2357: (another built-in register)
        !          2358: is the distance to the next trap,
        !          2359: which we assume is at the bottom margin of the page.
        !          2360: If the diversion is large enough to go past the trap,
        !          2361: the
        !          2362: .CW .if
        !          2363: is satisfied, and
        !          2364: a
        !          2365: .CW .bp
        !          2366: is issued.
        !          2367: In either case, the diverted output is then brought back with
        !          2368: .CW .XX .
        !          2369: It is essential to bring it back in no-fill mode so
        !          2370: .IT troff
        !          2371: will do no further processing on it.
        !          2372: .PP
        !          2373: This is not the most general keep-release,
        !          2374: nor is it robust in the face of all conceivable inputs,
        !          2375: but it would require more space than we have here to write it
        !          2376: in full generality.
        !          2377: This section is not intended
        !          2378: to teach everything about diversions,
        !          2379: but to sketch out enough that you can read
        !          2380: existing macro packages with some comprehension.
        !          2381: .NH
        !          2382: Acknowledgements
        !          2383: .PP
        !          2384: I am deeply indebted to the late J. F. Ossanna,
        !          2385: the author of
        !          2386: .IT troff ,
        !          2387: for his repeated patient explanations
        !          2388: of
        !          2389: fine points,
        !          2390: and for his continuing willingness to
        !          2391: adapt
        !          2392: .IT troff
        !          2393: to make other uses easier.
        !          2394: .PP
        !          2395: I am also grateful to Jim Blinn, Ted Dolotta,
        !          2396: Doug McIlroy, Mike Lesk and Joel Sturman
        !          2397: for helpful comments on this paper.
        !          2398: .NH
        !          2399: References
        !          2400: .LP
        !          2401: |reference_placement
        !          2402: .sp 100
        !          2403: .BP
        !          2404: .1C
        !          2405: .........
        !          2406: .SH
        !          2407: Appendix A: Character Set
        !          2408: .SP .5
        !          2409: .PP
        !          2410: Different output devices support different character sets,
        !          2411: although there is considerable commonality.
        !          2412: To get the one on the left, type the
        !          2413: four-character name on the right.
        !          2414: .TS
        !          2415: center;
        !          2416: l l6fCW l l6fCW l l6fCW l l6fCW.
        !          2417: \(ff   \e(ff   \(fi    \e(fi   \(fl    \e(fl   \(Fi    \e(Fi   \(Fl    \e(Fl
        !          2418: \(ru   \e(ru   \(em    \e(em   \(14    \e(14   \(12    \e(12   \(34    \e(34
        !          2419: \(co   \e(co   \(de    \e(de   \(dg    \e(dg   \(fm    \e(fm   \(ct    \e(ct   
        !          2420: \(rg   \e(rg   \(bu    \e(bu   \(sq    \e(sq   \(hy    \e(hy
        !          2421: .TE
        !          2422: .PP
        !          2423: The following are special-font characters:
        !          2424: .TS
        !          2425: center;
        !          2426: l l6fCW l l6fCW l l6fCW l l6fCW.
        !          2427: \(pl   \e(pl   \(mi    \e(mi   \(mu    \e(mu   \(di    \e(di   
        !          2428: \(eq   \e(eq   \(==    \e(==   \(>=    \e(>=   \(<=    \e(<=   
        !          2429: \(!=   \e(!=   \(+-    \e(+-   \(no    \e(no   \(sl    \e(sl   
        !          2430: \(ap   \e(ap   \(~=    \e(~=   \(pt    \e(pt   \(gr    \e(gr   
        !          2431: \(->   \e(->   \(<-    \e(<-   \(ua    \e(ua   \(da    \e(da   
        !          2432: \(is   \e(is   \(pd    \e(pd   \(if    \e(if   \(sr    \e(sr   
        !          2433: \(sb   \e(sb   \(sp    \e(sp   \(cu    \e(cu   \(ca    \e(ca   
        !          2434: \(ib   \e(ib   \(ip    \e(ip   \(mo    \e(mo   \(es    \e(es   
        !          2435: \(aa   \e(aa   \(ga    \e(ga   \(ci    \e(ci   \(L1    \e(L1   
        !          2436: \(sc   \e(sc   \(dd    \e(dd   \(lh    \e(lh   \(rh    \e(rh   
        !          2437: \(lt   \e(lt   \(rt    \e(rt   \(lc    \e(lc   \(rc    \e(rc   
        !          2438: \(lb   \e(lb   \(rb    \e(rb   \(lf    \e(lf   \(rf    \e(rf   
        !          2439: \(lk   \e(lk   \(rk    \e(rk   \(bv    \e(bv   \(ts    \e(ts   
        !          2440: \(br   \e(br   \(or    \e(or   \(ul    \e(ul   \(rn    \e(rn   
        !          2441: \(**   \e(**
        !          2442: .TE
        !          2443: .PP
        !          2444: These
        !          2445: four
        !          2446: characters also have two-character names. 
        !          2447: The \' is the apostrophe on terminals;
        !          2448: the \` is the other quote mark.
        !          2449: .TS
        !          2450: center;
        !          2451: l l6fCW l l6fCW l l6fCW l l6fCW.
        !          2452: \'     \e\(aa  \`      \e\(ga  \(mi    \e\(mi  \(em    \e(em
        !          2453: .TE
        !          2454: .PP
        !          2455: For greek, precede the roman letter by
        !          2456: .CW \e(*
        !          2457: to get the corresponding greek;
        !          2458: for example, 
        !          2459: .CW \e(*a
        !          2460: is
        !          2461: \(*a.
        !          2462: .sp
        !          2463: .in 0.5i
        !          2464: .nf
        !          2465: .cs R 36
        !          2466: abgdezyhiklmncoprstufxqw
        !          2467: \(*a\(*b\(*g\(*d\(*e\(*z\(*y\(*h\(*i\(*k\(*l\(*m\(*n\(*c\(*o\(*p\(*r\(*s\(*t\(*u\(*f\(*x\(*q\(*w
        !          2468: .sp
        !          2469: ABGDEZYHIKLMNCOPRSTUFXQW
        !          2470: \(*A\(*B\(*G\(*D\(*E\(*Z\(*Y\(*H\(*I\(*K\(*L\(*M\(*N\(*C\(*O\(*P\(*R\(*S\(*T\(*U\(*F\(*X\(*Q\(*W
        !          2471: .cs R
        !          2472: .in 0
        !          2473: .fi
        !          2474: .BP
        !          2475: .2C
        !          2476: .SH
        !          2477: Appendix B: Some Common Fonts
        !          2478: .LP
        !          2479: The following fonts are currently available on our Laserwriter II printers.
        !          2480: The magic
        !          2481: .I troff
        !          2482: short name is given as well as the name in the font catalog.
        !          2483: .de XX
        !          2484: .KS
        !          2485: .SP .5
        !          2486: \f1\\$1 (\f(CW\\$2\f1)
        !          2487: .br
        !          2488: .nf
        !          2489: .ft \\$2
        !          2490: abcdefghijklmnopqrstuvwxyz 0123456789
        !          2491: ABCDEFGHIJKLMNOPQRSTUVWXYZ
        !          2492: .ft
        !          2493: .KE
        !          2494: ..
        !          2495: .XX AvantGarde-BookOblique AI
        !          2496: .XX AvantGarde-Book AR
        !          2497: .XX AvantGarde-DemiOblique AX
        !          2498: .XX AvantGarde-Demi AB
        !          2499: .XX Bookman-DemiItalic KX
        !          2500: .XX Bookman-Demi KB
        !          2501: .XX Bookman-LightItalic KI
        !          2502: .XX Bookman-Light KR
        !          2503: .XX Courier-BoldOblique CX
        !          2504: .XX Courier-Bold CB
        !          2505: .XX Courier-Oblique CI
        !          2506: .XX Courier CW
        !          2507: .XX Helvetica-BoldOblique HX
        !          2508: .XX Helvetica-Bold HB
        !          2509: .XX Helvetica-Narrow-BoldOblique Hx
        !          2510: .XX Helvetica-Narrow-Bold Hb
        !          2511: .XX Helvetica-Narrow-Oblique Hi
        !          2512: .XX Helvetica-Narrow Hr
        !          2513: .XX Helvetica-Oblique HI
        !          2514: .XX Helvetica H
        !          2515: .XX NewCenturySchlbk-BoldItalic NX
        !          2516: .XX NewCenturySchlbk-Bold NB
        !          2517: .XX NewCenturySchlbk-Italic NI
        !          2518: .XX NewCenturySchlbk-Roman NR
        !          2519: .XX Palatino-BoldItalic PX
        !          2520: .XX Palatino-Bold PB
        !          2521: .XX Palatino-Italic PI
        !          2522: .XX Palatino-Roman PA
        !          2523: .XX Times-BoldItalic BI
        !          2524: .XX Times-Bold B
        !          2525: .XX Times-Italic I
        !          2526: .XX Times-Roman R
        !          2527: .XX ZapfChancery-MediumItalic ZI
        !          2528: .ig
        !          2529: show dingbats here
        !          2530: ..

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