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


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