title: extending scala collections with new attribute
body: <p>If I have a new collection with an additional attribute, e.g. :</p>
  
  <pre><code>class NewColl(var name: String, var values: Vector[Int])
      extends IndexedSeq[Int] with IndexedSeqLike[Int, NewColl] {...} 
  </code></pre>
  
  <p>how do I define <code>canBuildFrom</code> and <code>newBuilder</code> (cf. <a href="http://www.scala-lang.org/docu/files/collections-api/collections-impl.html" rel="nofollow">here</a>) such that if:</p>
  
  <pre><code>var v1 = NewColl("foo",Vector(1,2,3))
  var v2 = v1 filter(_ &gt; 1)
  var v3 = v1 map(_ + 1)
  </code></pre>
  
  <p>then <code>v2.name=="foo"</code> and <code>v3.name=="foo"</code>?  </p>