Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  a : A
  ls3 : list A
  X : All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) []) []
        (a :: ls3)
  ============================
   (All
      (λ z : B,
         All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z) []
           (a :: ls3)) [])


DEBUG:Going to execute:
subst