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


DEBUG:Going to execute:
match goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end