'Load and return the top-level name index (toplevelname_index).'

@property
def toplevelname_index(self):

    if (self._toplevelname_index is None):
        idxpath = join(self.base_dir, 'toplevelname_index')
        self._toplevelname_index = self.db.load_pickle(idxpath, {})
    return self._toplevelname_index
