Goal:
    A : Type  v : A
  ============================
   (∀ (a : A) (l : list A),
      (∃ n : nat, hd_error (skipn n l) = Some v) ↔ In v l
      → (∃ n : nat, hd_error (skipn n (a :: l)) = Some v) ↔ a = v ∨ In v l)


DEBUG:Going to execute:
trivial; hnf