Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  a : B
  a0 : A
  ls2 : list A
  a1 : A
  ls3 : list A
  X1 : All
         (λ z : B,
            All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z)
              (a0 :: ls2) (a1 :: ls3)) []
  X : All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y a) ls2 ls3
  X2 : P ls2 ls3 a0 a1 a
  ============================
   (All2_fold (λ (Γ Γ' : list A) (d d' : A), All (P Γ Γ' d d') []) ls2 ls3)


DEBUG:Going to execute:
lazymatch x with
           | [] => idtac
           | _ :: _ => idtac
           end