def _get_hosts(self, patterns):
        """
        finds hosts that match a list of patterns. Handles negative
        matches as well as intersection matches.
        """

        hosts = set()
        for p in patterns:
            if p.startswith("!"):
                # Discard excluded hosts
                hosts.difference_update(self.__get_hosts(p))
            elif p.startswith("&"):
                # Only leave the intersected hosts
                hosts.intersection_update(self.__get_hosts(p))
            else:
                # Get all hosts from both patterns
                hosts.update(self.__get_hosts(p))
        return hosts
finds hosts that match a list of patterns. Handles negative
        matches as well as intersection matches.