上下文切换

发布 context.become（替换或添加新行为）的两种可能方式是单独提供的，以实现嵌套接收的无杂波符号：

val a = actor(new Act {
  become { // this will replace the initial (empty) behavior
    case "info" ⇒ sender() ! "A"
    case "switch" ⇒
      becomeStacked { // this will stack upon the "A" behavior
        case "info"   ⇒ sender() ! "B"
        case "switch" ⇒ unbecome() // return to the "A" behavior
      }
    case "lobotomize" ⇒ unbecome() // OH NOES: Actor.emptyBehavior
  }
})