MAYBE

Problem:
 isList(nil()) -> tt()
 isList(Cons(x,xs)) -> isList(xs)
 downfrom(0()) -> nil()
 downfrom(s(x)) -> Cons(s(x),downfrom(x))
 f(x) -> cond(isList(downfrom(x)),s(x))
 cond(tt(),x) -> f(x)

Proof:
 Open