Goal:
    A : Type  l : list A  hd : A  tl : list A
  ============================
   (nth_error (hd :: tl) 0 = Some hd ∧ skipn 1 (hd :: tl) = tl)


DEBUG:Going to execute:
simpl
DEBUG LTAC Evaluated term: skipn_0
DEBUG LTAC Evaluated term: skipn_0