MAYBE

Problem:
 numbers() -> d(0())
 d(x) -> if(le(x,nr()),x)
 if(true(),x) -> cons(x,d(s(x)))
 if(false(),x) -> nil()
 le(0(),y) -> true()
 le(s(x),0()) -> false()
 le(s(x),s(y)) -> le(x,y)
 nr() -> ack(s(s(s(s(s(s(0())))))),0())
 ack(0(),x) -> s(x)
 ack(s(x),0()) -> ack(x,s(0()))
 ack(s(x),s(y)) -> ack(x,ack(s(x),y))

Proof:
 Open