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,y))
 gcd(s(x),s(y)) -> gcd(minus(max(x,y),min(x,transform(y))),s(min(x,y)))
 transform(x) -> s(s(x))
 transform(cons(x,y)) -> cons(cons(x,x),x)
 transform(cons(x,y)) -> y
 transform(s(x)) -> s(s(transform(x)))
 cons(x,y) -> y
 cons(x,cons(y,s(z))) -> cons(y,x)
 cons(cons(x,z),s(y)) -> transform(x)

Proof:
 Open