Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  b : B
  a : A
  ls2 : list A
  X0 : All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y b) (a :: ls2) []
  X1 : All
         (λ z : B,
            All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z)
              (a :: ls2) []) []
  ============================
   (All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) [b])
      (a :: ls2) [])


DEBUG:Going to execute:
first
[ All2_All_swap_t_step
| let H1 := fresh in
  let H2 := fresh in
  eapply All2_fold_impl;
   [ eapply All2_fold_prod
   | intros * [H1 H2]; constructor; (first [ exact H1 | exact H2 ]) ]
| let H1 := fresh in
  let H2 := fresh in
  eapply All2_fold_impl; [ eapply IH |  ]; clear IH ]