Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Q : list A → list A → A → A → Type
  h : All2_fold Q [] []
  ============================
   (let H := CoreTactics.block in
    All2_fold (λ (Γ Δ : list A) (x y : A), P Γ Δ x y × Q Γ Δ x y) [] [])


DEBUG:Going to execute:
CoreTactics.revert_until H
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            All2_fold
                              (λ (Γ Δ : list A) (x y : A),
                                 P Γ Δ x y × Q Γ Δ x y) [] [])