Goal:
  
  A : Type
  n : nat
  hd : A
  tl : list A
  IHn : ∀ (l : list A) (hd : A) (tl : list A),
          skipn n l = hd :: tl → nth_error l n = Some hd ∧ skipn (S n) l = tl
  ============================
   ([] = hd :: tl → nth_error [] (S n) = Some hd ∧ skipn (S (S n)) [] = tl)


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>
DEBUG LTAC Evaluated term: skipn_nil
DEBUG LTAC Evaluated term: skipn_nil