MAYBE

Problem:
 le(0(),y) -> true()
 le(s(x),0()) -> false()
 le(s(x),s(y)) -> le(x,y)
 int(x,y) -> if(le(x,y),x,y)
 if(true(),x,y) -> cons(x,int(s(x),y))
 if(false(),x,y) -> nil()

Proof:
 Open