Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Q : list A → list A → A → A → Type
  h1' : list A
  h1 : list A
  h : All2_fold Q (pr1 {| pr1 := h1'; pr2 := h1 |})
        (pr2 {| pr1 := h1'; pr2 := h1 |})
  ============================
   ({| pr1 := []; pr2 := [] |} = {| pr1 := h1'; pr2 := h1 |}
    → 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:
DepElim.decompose_exists xn id