Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  a : A
  ls2 : list A
  X : 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) []) 
      (a :: ls2) [])%type


DEBUG:Going to execute:
subst