Trait as Stackable modifications

HOge.scala

// Either is OK.
// abstract Base { def say(word: String) }
trait Base { def say(word: String) }

class Hoge extends Base { def say(word: String) = println(word) }

trait Smile extends Base { abstract override def say(word: String) = super.say(word + " :)") }

trait Hello extends Base { abstract override def say(word: String) = super.say("Hello " + word) }

object Hoge extends App {
  val hoge = new Hoge with Smile with Hello
  hoge.say("World")
}

$ scalac Hoge.scala
$ scala Hoge
Hello World :)