Goal:
  
  A : Type
  P : nat → A → Type
  n : nat
  x : A
  Γ : list A
  IHΓ : All_fold (λ (Γ : list A) (d : A), P (n + #|Γ|) d) (List.rev Γ)
        → Alli P n Γ
  a : All_fold (λ (Γ : list A) (d : A), P (n + #|Γ|) d) (List.rev Γ)
  p : P (n + #|List.rev Γ|) x
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in Alli P n (Γ ++ [x]))


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in Alli P n (Γ ++ [x]))