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


DEBUG:Going to execute:
exists (S n)
DEBUG LTAC Evaluated term: 0