Goal:
  
  A : Type
  P : nat → A → Type
  x : A
  l : list A
  k : nat
  IHl : Alli (λ k' : nat, P (Nat.pred #|l| - k' + k)) 0 (List.rev l)
  a : Alli P k l
  b : Alli P (#|l| + k) [x]
  n : nat
  x0 : A
  X : P (Nat.pred #|l| - n + k) x0
  ============================
   (Nat.pred #|l| - n = Nat.pred (#|l| + 1) - S n)


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