Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  c : C
  a : A
  ls2 : list A
  X0 : All2 (λ (x : A) (y : B), P x y c) (a :: ls2) []
  X1 : All (λ z : C, All2 (λ (x : A) (y : B), P x y z) (a :: ls2) []) []
  ============================
   (All2 (λ (x : A) (y : B), All (P x y) [c]) (a :: ls2) [])


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 ]