New Rhinocommon Release Does Not Now Include Rtree Class

Does anyone know why the RTree class is no longer supported in teh latest release of rhinocommon?

The online SDK says that is part of the Rhino.Geometry namespace but it defintley does not now exist when i t try to recompile legacy code against this release of rhinocommon. As expected it is also not called out when using intellisense in the VS2010 IDE.

Why is this class gone? WHere is it now?