def otherwise=true:

proc xxtoy(chan in, out, value x, y)=
    var c:
    seq
	c:= not EOF
	while c<>EOF
	    seq
		in ? c
		if
		    c=x
			seq
			    in ? c
			    if
				c=x
				    out ! y
				otherwise
				    out ! x; c
		    otherwise
			out ! c
:
chan link:
par
    xxtoy(input, link, 'a', 'b')
    xxtoy(link, output, 'b', 'c')
