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'1' : list A
  H'1 : list A
  ============================
   (All2_fold P H'1' H'1
    → {| pr1 := a :: Δ ++ Γ; pr2 := a0 :: Δ' ++ Γ' |} =
      {| pr1 := H'1'; pr2 := H'1 |}
      → 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:
intros p