Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Q : list A → list A → A → A → Type
  h : All2_fold Q [] []
  h0 := {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := h |}
     : sigma
         (λ index : sigma (λ _ : list A, list A),
            All2_fold Q (pr1 index) (pr2 index))
  ============================
   (let H := CoreTactics.block in
    let H0 := 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:
set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2