trait Events[+A] { outer =>
  private[this] val listeners = Set[A => Unit]()

  def observe(f: A => Unit) {
    listeners += f
  }

  def merge[B >: A](that: Events[B]) = new Events[B] {
    override def observe(f: B => Unit) {
      outer.observe(f)
      that.observe(f)
    }
  }

  def allListeners[B <: A]: List[B => Unit] = listeners.toList
}

trait EventSource[T] extends Events[T] {
  def raise(x: T) {
    allListeners.foreach(_(x))
  }
}