MAYBE

Problem:
 strict:
  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))
  -(x,0()) -> x
  -(s(x),s(y)) -> -(x,y)
  gcd(s(x),s(y)) -> gcd(-(max(x,y),min(x,y)),s(min(x,y)))
  gcd(s(x),0()) -> s(x)
  gcd(0(),s(y)) -> s(y)
 weak:
  min(x,y) -> min(y,x)
  max(x,y) -> max(y,x)

Proof:
 Open