Using the extends keyword in a companion object definition

Question

I am reading this article by Odersky and Spoon. So, here is the code 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 a companion object PrefixMap extends {...} as an object PrefixMap extends {...} and not just an object PrefixMap {...} ? Could you hint that in this case it is possible to expand?