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
  ============================
   (skipn (S n) [] = hd :: tl
    → nth_error [] (S n) = Some hd ∧ skipn (S (S n)) [] = tl)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
DEBUG LTAC Evaluated term: l