Goal:
  
  A : Type
  P : list A → A → Type
  Q : list A → list A → A → A → Type
  Γ' : list A
  H1' : list A
  H1 : list A
  H : All2_fold Q (pr1 {| pr1 := H1'; pr2 := H1 |})
        (pr2 {| pr1 := H1'; pr2 := H1 |})
  ============================
   ({| pr1 := []; pr2 := Γ' |} = {| pr1 := H1'; pr2 := H1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2_fold (λ (Γ Γ' : list A) (d d' : A), P Γ d × Q Γ Γ' d d') [] Γ')


DEBUG:Going to execute:
DepElim.decompose_exists xn id