Goal:
  
  A : Type
  B : Type
  f : A → B
  IHn : nat
  l : ∀ l : list A,
        nth_error (map f l) (S IHn) = option_map f (nth_error l (S IHn))
  ============================
   (match map f [] with
    | _ :: _ :: l0 => nth_error l0 IHn
    | _ => None
    end = option_map f None)


DEBUG:Going to execute:
trivial; hnf; intros; (solve
            [ do
            ![ solve
             [ trivial | apply : eq_sym ; trivial ]
             | discriminate
             | contradiction
             | Ltac.split ]
            | case not_locked_false_eq_true; Ltac.assumption
            | match goal with
              | H:¬ _ |- _ => solve [ case H; trivial ]
              end ])