Goal:
    A : Type  B : Type  f : A → B  l : nat
  ============================
   ((∀ l0 : list A, map f (skipn l l0) = skipn l (map f l0))
    → ∀ l0 : list A, map f (skipn (S l) l0) = skipn (S l) (map f l0))


DEBUG:Going to execute:
trivial; hnf; intros