Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  p : A → B → bool
  Hp : ∀ (x : A) (y : B), reflect (P x y) (p x y)
  ============================
   (true → Forall2 P [] [])


DEBUG:Going to execute:
trivial; hnf; intros