Goal:
  
  A : Type
  n : nat
  IHn : ∀ l : list A, #|l| ≤ n → skipn n l = []
  a : A
  l : list A
  h : S #|l| ≤ S n
  ============================
   (#|l| ≤ n)


DEBUG:Going to execute:
intros; Zify.zify_pre_hook; zify_elim_let; zify_op