MAYBE

Problem:
 min(x,0()) -> 0()
 min(0(),y) -> 0()
 min(s(x),s(y)) -> s(min(x,y))
 max(x,0()) -> x
 max(0(),y) -> y
 max(s(x),s(y)) -> s(max(x,y))
 minus(x,0()) -> x
 minus(s(x),s(y)) -> s(minus(x,any(y)))
 gcd(s(x),s(y)) -> gcd(minus(max(x,y),min(x,y)),s(min(x,y)))
 any(s(x)) -> s(s(any(x)))
 any(x) -> x

Proof:
 Open