How to get None when reduction was not applied and Some(value) when it did in scala.rx

gistfile1.scala

package frp

import rx._
import rx.ops._

object Demo extends App {

  case class Val[+T](value: T, reduced: Boolean = false) {
    def toOpt = if (reduced) Some(value) else None
  }

  implicit class MyRxOps[+T](val source: Rx[T]) extends AnyVal{
    def reducedOnly[A >: T](combiner: (A, A) => A): Rx[Option[A]] = {
      source.map(Val(_)) reduce { (a: Val[A], b: Val[A]) =>
        Val(combiner(a.value, b.value), reduced = true)
      } map (_.toOpt)
    }
  }

  val price = Var(1)
  val deltas = price.reducedOnly((oldVal, newVal) => newVal - oldVal)

  println(deltas()) // there was only one price, so deltas should be None
  price() = 2
  println(deltas()) // price went up by 1
  price() = 3
  println(deltas()) // price went up by 1

}