Goal:
  
  A : Type
  a : A
  l : list A
  IHl : ∀ n : nat, #|l| ≤ n → firstn n l = l
  H : S #|l| ≤ 0
  ============================
   ([] = a :: l)


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