Thomas Neidhart of SpaceApplications has implemented the Toma query language on top of Ontopia. Toma is a query language for Topic Maps designed by Rani Pinchuk of SpaceApplications. The implementation is currently in the sandbox part of Subversion, and not part of Ontopia proper, but the wiki page explains how to check it out and run it. Currently, Toma queries can only be run using a command-line client (or the API), but work is currently under way to make Toma available in the Omnigator query plug-in.
Ontopia's Blog 