@ Cons x xs ys Cons x @ xs ys @ Nil ys ys binom Cons x xs Cons x' xs' @ binom xs xs' binom xs Cons x' xs' binom Cons x xs Nil Cons Nil Nil binom Nil k Cons Nil Nil goal x y binom x y @ 2 Cons 2 Nil 0 binom 2 goal 2 INNERMOST