def _search_capability(self, base):
        """Given a class, return a list of all of the derived classes that
        are themselves derived from Capability."""
        if _debug: Collector._debug("_search_capability %r", base)

        rslt = []
        for cls in base.__bases__:
            if issubclass(cls, Collector):
                map( rslt.append, self._search_capability(cls))
            elif issubclass(cls, Capability):
                rslt.append(cls)
        if _debug: Collector._debug("    - rslt: %r", rslt)

        return rslt
Given a class, return a list of all of the derived classes that
        are themselves derived from Capability.