Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  a : C
  ls1 : list C
  IH : ∀ (ls2 : list A) (ls3 : list B),
         All2 (λ (x : A) (y : B), All (P x y) ls1) ls2 ls3
         → All (λ z : C, All2 (λ (x : A) (y : B), P x y z) ls2 ls3) ls1
  a0 : A
  ls2 : list A
  b : B
  ls3 : list B
  x : A
  y : B
  l : list A
  l' : list B
  X0 : All (P a0 b) (a :: ls1)
  X1 : All2 (λ (x : A) (y : B), All (P x y) (a :: ls1)) ls2 ls3
  H0 : x = a0
  H1 : l = ls2
  H : y = b
  H3 : l' = ls3
  ============================
   (All (λ z : C, All2 (λ (x : A) (y : B), P x y z) (a0 :: ls2) (b :: ls3))
      (a :: ls1))


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 ]