Goal:
  
  A : Type
  a : A
  l : list A
  l' : list A
  t : A
  IHl : ∀ n : nat, nth_error l n = Some t → nth_error (l ++ l') n = Some t
  ============================
   (Some a = Some t → Some a = Some t)


DEBUG:Going to execute:
congruence