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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def all_hosts(self):
 """List of hosts, passives, and arbiters known to this server."""

        return set(imap(common.clean_node, itertools.chain(
            self._doc.get('hosts', []),
            self._doc.get('passives', []),
            self._doc.get('arbiters', []))))