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


DEBUG:Going to execute:
trivial; hnf; intros