Scala特征的伴随对象

Question

我有这样的事情：

trait AO extends A {
  private var link: String = "AO"
  //...
}
trait AR extends A {
  private var link: String = "AR"
  //...
}    

object AO extends AO with Atr {
  atr += link
}

object AR extends AR with Atr {
  atr += link  
}

trait Atr {
  import scala.collection.mutable._
  val atr: ListBuffer[String] = ListBuffer()
  def echo() = {
    for(f <- atr) println("attribute: " + f)
  }
}

object TestF {
  def main(args: Array[String]) {
    AR.echo //not what I want!
    AO.echo //not what I want!
    echo() //doesn't show nothing...
  }
}

我如何在ListBuffer中添加这些属性，比如atr，并像TestF的第三行一样一次性读取它们？