Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  a : A
  ls2 : list A
  ============================
   ([] = [])


DEBUG:Going to execute:
let is_ctor_list x :=
            lazymatch x with
            | [] => idtac
            | _ :: _ => idtac
            end
           in
           match goal with
           | H:All2 _ ?x ?y
             |- _ =>
                 (first [ is_ctor_list x | is_ctor_list y ]); inversion H;
                  clear H
           | H:All2_fold _ ?x ?y
             |- _ =>
                 (first [ is_ctor_list x | is_ctor_list y ]); inversion H;
                  clear H
           | H:All _ ?x |- _ => is_ctor_list x; inversion H; clear H
           | |- (_ :: _ = []) + _ => right
           | |- (?x = ?x) + _ => left
           end
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.