h(0()) -> s(0())
s(s(x)) -> x
f(0(),x) -> x
g(0(),x) -> x
f(s(x),y) -> s(f(x,y))
f(g(x,y),0()) -> g(s(x),y)
g(s(0()),x) -> f(x,0())
f(f(x,0()),0()) -> x