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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 ret_nodegroup_minions(self):
        '''
        Return minions which match the special list-only groups defined by
        ssh_list_nodegroups
        '''
        nodegroup = __opts__.get('ssh_list_nodegroups', {}).get(self.tgt, [])
        nodegroup = _tgt_set(nodegroup)
        return self._ret_minions(nodegroup.intersection)