[boost] copy on write for std containers

I wrote a couple of lines and replaced std::set and std::map with some copy-on-write-wrappers and I got a dramatic speed improvement with the last gcc 4.5. I think. My code was using recursive functions iterating over a tree returning and setting such objects. I looked and it seems there is nothing like this in boost! Ideally this should be an template argument for the std containers. Peter