Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  a : A
  ls2 : list A
  b : B
  ls3 : list B
  ============================
   ([] = [])


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 IH' := fresh in
    eauto; edestruct IH as [IH'| IH']; try (apply IH'; clear IH'); clear IH) ]