'Newest version for which all items are available?
:returns: the newest available version for which all members
(``cert, ``privkey``, ``chain``, and ``fullchain``) exist
:rtype: int'

def latest_common_version(self):

    versions = [self.available_versions(x) for x in ALL_FOUR]
    return max((n for n in versions[0] if all(((n in v) for v in versions[1:]))))
