Goal:
    A : Type  B : Type  C : Type  P : A → B → C → Type
  ============================
   (All (λ z : C, All2 (λ (x : A) (y : B), P x y z) [] []) []
    → ([] = []) + All2 (λ (x : A) (y : B), All (P x y) []) [] [])


DEBUG:Going to execute:
progress intros