nil :: list
cons :: A -> list -> list

skip :: Int -> list -> list
skip(k, nil) -> nil
skip(k, ys) -> ys | k <= 0
skip(k, cons(x, ys)) -> skip(k-1, ys) | k > 0

skipfold :: (A -> B -> B) -> B -> Int -> list -> B
skipfold(F, n, k, nil) -> n
skipfold(F, n, k, cons(x, ys)) -> skipfold(F, F(x, n), k, skip(k, ys))

zip :: list -> list -> list
zip(nil, ys) -> ys
zip(cons(x, xs), ys) -> cons(x, zip(ys, xs))