'Recalculate the _versions set.
Re-fetch the local (server) versions and expand with consumers\'
versions.'

def _recalculate_versions(self):

    versions = self._get_local_resource_versions()
    for versions_dict in self._versions_by_consumer.values():
        for (res_type, res_version) in versions_dict.items():
            versions[res_type].add(res_version)
    self._versions = versions
