Usage of extends keyword in a companion object definition

Question

I am reading this article by Odersky and Spoon. So here goes a code listing from this article with one peculiar aspect:

import scala.collection.mutable.{Builder, MapBuilder}
import scala.collection.generic.CanBuildFrom

object PrefixMap extends {
  def empty[T] = new PrefixMap[T]

  def apply[T](kvs: (String, T)*): PrefixMap[T] = {
    val m: PrefixMap[T] = empty
    for (kv <- kvs) m += kv
    m
  }

  def newBuilder[T]: Builder[(String, T), PrefixMap[T]] = 
    new MapBuilder[String, T, PrefixMap[T]](empty)

  implicit def canBuildFrom[T]
    : CanBuildFrom[PrefixMap[_], (String, T), PrefixMap[T]] = 
      new CanBuildFrom[PrefixMap[_], (String, T), PrefixMap[T]] {
        def apply(from: PrefixMap[_]) = newBuilder[T]
        def apply() = newBuilder[T]
      }
}

Why do they declare the companion object as object PrefixMap extends {...} and not just simply object PrefixMap {...} ? Could you hint on the purpose of extending in this case?