Goal:
  
  A : Type
  a : A
  l : list A
  n : nat
  IHl : ∀ n : nat, #|l| ≤ n → firstn n l = l
  H : S #|l| ≤ S n
  ============================
   (#|l| ≤ n)


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