MAYBE

Problem:
 digits() -> d(0())
 d(x) -> if(le(x,s(s(s(s(s(s(s(s(s(0())))))))))),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)

Proof:
 Open