MAYBE

Problem:
 f(x,x) -> g()
 a() -> b()
 a() -> c()
 b() -> b()
 c() -> c()
 f(a(),x) -> g()
 f(b(),x) -> g()
 f(c(),x) -> g()
 f(x,a()) -> g()
 f(x,b()) -> g()
 f(x,c()) -> g()

Proof:
 Open