Goal:
    A : Type  B : Type  C : Type  P : A → B → C → Type  c : C
  ============================
   (All2 (λ (x : A) (y : B), All (P x y) [c]) [] [])


DEBUG:Going to execute:
first
[ All2_All_swap_t_step
| let H1 := fresh in
  let H2 := fresh in
  eapply All2_impl;
   [ eapply All2_prod
   | intros * [H1 H2]; constructor; (first [ exact H1 | exact H2 ]) ]
| let H1 := fresh in
  let H2 := fresh in
  eapply All2_impl; [ eapply IH |  ]; clear IH ]