Goal:
  
  A : Type
  a : A
  l : list A
  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)
  X : P (#|List.rev l| + 0) a
  ============================
   (nth_error (a :: l) 0 = Some x → P (#|l| - 0) x)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2
DEBUG LTAC Evaluated term: n

File "./theories/All_Forall.v", line 1046, characters 27-42:
Warning: Notation List.rev_length is deprecated since 8.20.
Use length_rev instead.
[deprecated-syntactic-definition-since-8.20,deprecated-since-8.20,deprecated-syntactic-definition,deprecated,default]