Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  a : C
  ls1 : list C
  IH : ∀ (ls2 : list A) (ls3 : list B),
         All (λ z : C, All2 (λ (x : A) (y : B), P x y z) ls2 ls3) ls1
         → (ls1 = []) + 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) [] []) (a :: ls1)
    → (a :: ls1 = []) +
      All2 (λ (x : A) (y : B), All (P x y) (a :: ls1)) [] [])


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) ]
Level 0: <in exception printer>:
Anomaly
"File "tactics/tacticals.ml", line 190, characters 68-74: Assertion failed."
Please report at http://coq.inria.fr/bugs/.
<original exception>: Backtrace:

Uncaught exception Tacticals.FailError(0, _).
Level 0: <in exception printer>:
Anomaly
"File "tactics/tacticals.ml", line 190, characters 68-74: Assertion failed."
Please report at http://coq.inria.fr/bugs/.
<original exception>: Backtrace:

Uncaught exception Tacticals.FailError(0, _).