MAYBE

Problem:
 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))

Proof:
 Open