Goal:
  
  A : Type
  v : A
  a : A
  l : list A
  H : (∃ 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