(from AG01 3.49)
(VAR x y)
(RULES
f(c(s(x),y)) -> f(c(x,s(y)))
f(c(s(x),s(y))) -> g(c(x,y))
g(c(x,s(y))) -> g(c(s(x),y))
g(c(s(x),s(y))) -> f(c(x,y)))