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


DEBUG:Going to execute:
apply iff_reflect;
            change (forallb p l = true) with (forallb p l : Prop)