[collections] reference counting

[Torsten Curdt <tc...@apache.org>]

In the back of my head I thought this would
be possible already ...but right now  I just
cannot find how :-/

I would like to have a Set that does
reference counting. Example:

 set.add("a") // 1x a
 set.add("a") // 2x a
 set.remove("a") // 1x a
 set.remove("a") // empty

A useful addition or just me not
seeing the obvious?

cheers
--
Torsten