Goal:
  
  A : Type
  a : A
  l : list A
  n : nat
  P : A → bool
  d : A
  IHl : n < #|l| → ∃ x : A, nth n l d = x ∧ P x
  pa : is_true (P a)
  pl : is_true (forallb P l)
  pn : S n < S #|l|
  ============================
   (n < #|l|)


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