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import scala.collection.immutable._
import scala.collection.mutable.ListBuffer
import scala.collection.generic._
trait Base[+A] extends Traversable[A] {
def add[B >: A, That](that: Traversable[B])(implicit bf: CanBuildFrom[Base[A], B, That]): That = {
val b = bf(this)
b ++= this
b ++= that
b.result
}
}
abstract class Derived[@specialized +A] extends Base[A] {
override def add[B >: A, That](that: Traversable[B])(implicit bf: CanBuildFrom[Base[A], B, That]): That = {
val b = bf(this)
super.add[B, That](that)
}
}
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