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


DEBUG:Going to execute:
subst