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


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