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)
  ============================
   (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:
lazymatch goal with
           | id':=?v:_
             |- context [ id ] =>
                 generalize (eq_refl : v = id'); clearbody id'; simpl in id';
                  cont id id' id v
           | id':=?v:_
             |- _ =>
                 let id'1 := fresh id' in
                 let id'2 := fresh id' in
                 set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
                  hnf in (value of id'1), (value of id'2);
                  try red in (type of id'2);
                  match goal with
                  | id'1:=?t:_
                    |- _ =>
                        generalize (eq_refl : t = id'1); clearbody id'2 id'1;
                         clear id' id; try unfold signature in id'2;
                         hnf in id'2; simpl in id'2; rename id'2 into id;
                         cont id id id'1 t
                  end
           end