Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  b : B
  a : B
  ls1 : list B
  IH : ∀ ls2 ls3 : list A,
         All
           (λ z : B,
              All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z) ls2 ls3)
           (b :: ls1)
         → All2_fold
             (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) (b :: ls1)) ls2
             ls3
  a0 : A
  ls2 : list A
  X0 : All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y b) (a0 :: ls2) []
  X1 : All
         (λ z : B,
            All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z)
              (a0 :: ls2) []) (a :: ls1)
  ============================
   (All2_fold
      (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) (b :: a :: ls1))
      (a0 :: ls2) [])


DEBUG:Going to execute:
idtac
Level 4: x
 has value 
(a0 :: ls2)