MAYBE

Problem:
 leq(0(),y) -> true()
 leq(s(x),0()) -> false()
 leq(s(x),s(y)) -> leq(x,y)
 if(true(),x,y) -> x
 if(false(),x,y) -> y
 -(x,0()) -> x
 -(s(x),s(y)) -> -(x,y)
 mod(0(),y) -> 0()
 mod(s(x),0()) -> 0()
 mod(s(x),s(y)) -> if(leq(y,x),mod(-(s(x),s(y)),s(y)),s(x))

Proof:
 Open