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 isList 1 nil 0 tt 0 Cons 2 downfrom 1 0 0 s 1 f 1 cond 2 FULL TRS//AProVE_10/downfrom.trs