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