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 : B
  l : list B
  X0 : All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y b) 
         (a :: ls2) (a0 :: ls3)
  X1 : All
         (λ z : B,
            All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z)
              (a :: ls2) (a0 :: ls3)) []
  H0 : x = b
  H1 : l = []
  ============================
   (All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) [b])
      (a :: ls2) (a0 :: ls3))


DEBUG:Going to execute:
progress subst
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.