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


DEBUG:Going to execute:
move  =>[->|[n H]]; [ exists 0 | exists (S n) ]