分别提供了两种发出context.become（替换或添加新行为）的可能方式，以使嵌套接收的符号更整洁：
val a = actor(new Act {
become { // 这将替代初始（空）行为
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
}
})