Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  c : C
  a : C
  ls1 : list C
  a0 : A
  ls2 : list A
  b : B
  ls3 : list B
  X : P a0 b c
  X2 : All2 (λ (x : A) (y : B), P x y c) ls2 ls3
  X3 : All
         (λ z : C, All2 (λ (x : A) (y : B), P x y z) (a0 :: ls2) (b :: ls3))
         ls1
  X1 : P a0 b a
  X4 : All2 (λ (x : A) (y : B), P x y a) ls2 ls3
  ============================
   (All2 (λ (x : A) (y : B), P x y c) ls2 ls3)


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 ]
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, _).