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


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 ])