.PS
.ps 20
## figure 12
B1:     box "\sf provider"
B2:     box invis with .sw at B1.n "SERVICE" "OFFERED" \
                height boxht*2 width boxwid*2
B3:     box "\sf consumer" with .s at B2.nw
##
        arrow from B1.n to B3.s
##
.PE