'Load and return the top-level prefix index (toplevelprefix_index).'

@property
def toplevelprefix_index(self):

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