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


DEBUG:Going to execute:
idtac
Level 4: x
 has value 
[b]