Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  a : B
  a0 : A
  ls2 : list A
  a1 : A
  ls3 : list A
  X : All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y a) ls2 ls3
  X2 : P ls2 ls3 a0 a1 a
  ============================
   (All2_fold (λ (Γ Γ' : list A) (d d' : A), All (P Γ Γ' d d') []) ls2 ls3)


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