Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Q : list A → A → Type
  d : A
  d' : A
  Γ : list A
  Γ' : list A
  X : All2_fold P Γ Γ'
  p : P Γ Γ' d d'
  IHX : All_fold Q Γ
        → All_fold Q Γ'
          → All2_fold
              (λ (Γ Γ' : list A) (x y : A), Q Γ x × Q Γ' y × P Γ Γ' x y) Γ Γ'
  l : All_fold Q Γ
  q : Q Γ d
  r : All_fold Q (d' :: Γ')
  r0 := {| pr1 := d' :: Γ'; pr2 := r |}
     : sigma (λ index : list A, All_fold Q index)
  r1 := d' :: Γ' : list A
  r2 : (λ index : list A, All_fold Q index) r1
  ============================
   (r = r2
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2_fold (λ (Γ Γ' : list A) (x y : A), Q Γ x × Q Γ' y × P Γ Γ' x y)
        (d :: Γ) (d' :: Γ'))


DEBUG:Going to execute:
clear id' id