Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  Γ' : list A
  a : A
  Δ : list A
  a0 : A
  Δ' : list A
  H : S #|Δ| = S #|Δ'|
  IHΔ : All2_fold P (Δ ++ Γ) (Δ' ++ Γ')
        → All2_fold P Γ Γ'
          × All2_fold (λ Δ Δ' : list A, P (Δ ++ Γ) (Δ' ++ Γ')) Δ Δ'
  H' : All2_fold P (Δ ++ Γ) (Δ' ++ Γ')
  p : P (Δ ++ Γ) (Δ' ++ Γ') a a0
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2_fold P Γ Γ'
    × All2_fold (λ Δ Δ' : list A, P (Δ ++ Γ) (Δ' ++ Γ')) (a :: Δ) (a0 :: Δ'))


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in
 All2_fold P Γ Γ'
 × All2_fold (λ Δ Δ' : list A, P (Δ ++ Γ) (Δ' ++ Γ')) (a :: Δ) (a0 :: Δ'))