Goal:
  
  A : Type
  a : A
  l : list A
  n : nat
  P : nat → A → Type
  x : A
  IHl : ∀ (n : nat) (x : A),
          Alli P 0 (List.rev l) → nth_error l n = Some x → P (#|l| - S n) x
  Alll : Alli P 0 (List.rev l)
  Alla : Alli P (#|List.rev l| + 0) [a]
  ============================
   (nth_error (a :: l) n = Some x → P (#|l| - n) x)


DEBUG:Going to execute:
fun H => inversion_clear H