It is a big problem to specify a fixed interface and implementation for something diverse like a tree. There are so many aspects and wishes that could be fulfilled. Jeff Mirwaisi bypassed that issue by extending the policy based approach. Since Jeff is moving soon, and has no time to follow the discussion I try to discuss in his name. Thus I uploaded treelib.tar.bz2 to the sf.net vault at. http://boost-sandbox.sourceforge.net/vault/index.php The tree type is composed out of a set of feature. A feature is a set of fragment, or a single fragment. The feature type list is specified by the user. `A fragment is a binary meta function that produces a step-wise refinement of a base type by adding, hiding,overloading or overriding member data and member functions resulting in an augmented version of the base.' [JM05]. The user is allowed to add his own fragments, so it is possible to change the internals even after the library has been ``shipped'' to the user. That is also the main difference to policy based design, which would define a fixed set of extension points. The library in its current state is not yet complete, erase methods, and proper rebalancing of the different tree variants is missing. Regards, Andreas Pokorny [JM05] gen - FOP Utility Library - Documentation of the feature oriented programming utility library, which is currently part of the treelib source code.