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


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 ]
DEBUG LTAC Evaluated term: ls3
DEBUG LTAC Evaluated term: ls2
DEBUG LTAC Evaluated term: ls1