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


DEBUG:Going to execute:
first
[ All2_All_swap_t_step
| let H1 := fresh in
  let H2 := fresh in
  eapply All_impl;
   [ eapply All_prod
   | intros ? [H1 H2]; constructor; (first [ exact H1 | exact H2 ]) ]; eauto;
   apply IH; clear IH ]