Goal:
    A : Type  v : A  _a_ : A  _l_ : list A  _n_ : nat
  ============================
   (hd_error (skipn _n_ _l_) = Some v
    → _a_ = v ∨ ∃ n : nat, hd_error (skipn n _l_) = Some v)


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 ])