Goal:
  
  A : Type
  P : list A → A → Type
  Q : list A → list A → A → A → Type
  d0 : A
  Γ1 : list A
  d : A
  Γ0 : list A
  X : All_fold P Γ0
  p : P Γ0 d
  IHX : ∀ Γ : list A,
          All2_fold Q Γ Γ0
          → All2_fold (λ (Γ0 Γ' : list A) (d d' : A), P Γ' d' × Q Γ0 Γ' d d')
              Γ Γ0
  H : All2_fold Q Γ1 Γ0
  q : Q Γ1 Γ0 d0 d
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2_fold (λ (Γ Γ' : list A) (d d' : A), P Γ' d' × Q Γ Γ' d d')
      (d0 :: Γ1) (d :: Γ0))


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in
 All2_fold (λ (Γ Γ' : list A) (d d' : A), P Γ' d' × Q Γ Γ' d d') 
   (d0 :: Γ1) (d :: Γ0))