scala default map updating

gistfile1.scala

import scala.collection.mutable.Map
import scala.collection.mutable.Buffer

val foo = Map[Int, Buffer[Int]]() withDefault ((key) => Buffer())

foo(1) = Buffer(1, 2)
foo(1) += 3
foo(1) // => ArrayBuffer(1, 2, 3)
/* This all seems as expected */

foo(2) // => ArrayBuffer()
foo(2) += 3
foo(2) // => ArrayBuffer()
/* This is a little annoying, but it makes sense when you think about it -- the withDefault
 * function doesn't have access to the map itself, so it can only produce a default value
 * (but not save it). Also, the behavior of the WithDefault seems to be that it doesn't save
 * the default value when accessed.
 *
 * My question is: is there a way to get this behavior, or do I have to implement it myself?
 * In ruby, for example, the constructor block has access to the hash as well as the key:
 *
 *   Hash.new { |hash, key| hash[key] = [] }
 *
 * Thanks!
 */