Goal:
  
  A : Type
  P : A → Prop
  Q : A → Prop
  p : A → bool
  l : list A
  HQ : Forall Q l
  Hp : ∀ x : A, Q x → reflect (P x) (p x)
  ============================
   (reflect (Forall P l) (forallb p l))


DEBUG:Going to execute:
apply iff_reflect