Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  b : B
  a : A
  ls2 : list A
  a0 : A
  ls3 : list A
  X : All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y b) ls2 ls3
  X2 : P ls2 ls3 a a0 b
  ============================
   (All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) [b]) 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 H1 := fresh in
  let H2 := fresh in
  eapply All2_fold_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, _).