.PS
.ps 20
## figure 3
B1:	box shaded
B2:	box invis with .s at (B1.n.x, B1.n.y+lineht) "$\hbox{O}_1$"
B3:	box invis with .n at (B1.s.x, B1.s.y-lineht) "$\hbox{I}_1$"
	arrow from B1.n to B2.s
	arrow from B3.n to B1.s
	arrow from B1.w to (B1.w.x-linewid, B1.w.y)
	move to (B1.w.x, B1.w.y+boxht/3); "$\hbox{P}_1$ " rjust
##
B4:	box shaded with .w at (B1.e.x+4*linewid, B1.e.y)
B5:	box invis with .s at (B4.n.x, B4.n.y+lineht) "$\hbox{O}_2$"
B6:	box invis with .n at (B4.s.x, B4.s.y-lineht) "$\hbox{I}_2$"
	arrow from B4.n to B5.s
	arrow from B6.n to B4.s
	arrow from B4.e to (B4.e.x+linewid, B4.e.y)
	move to (B4.e.x, B4.e.y+boxht/3); " $\hbox{P}_2$" ljust
##
.PE