Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  HP : ∀ (Γ Γ' Γ'' : list A) (x y z : A),
         All2_fold P Γ Γ'
         → All2_fold P Γ' Γ''
           → All2_fold P Γ Γ'' → P Γ Γ' x y → P Γ' Γ'' y z → P Γ Γ'' x z
  z : list A
  d : A
  d' : A
  Γ : list A
  Γ' : list A
  H : All2_fold P Γ Γ'
  p : P Γ Γ' d d'
  IHAll2_fold : ∀ z : list A, All2_fold P Γ' z → All2_fold P Γ z
  H' : All2_fold P (d' :: Γ') z
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All2_fold P (d :: Γ) z)


DEBUG:Going to execute:
fun id cont =>
             let id' := fresh id in
             get_signature_pack id id'; hnf in (value of id');
              hnf in (type of id');
              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