def get_label_by_id(self, id):
        '''
        Returns a subset label by a "region id". 
        A region id is a string '10', '01' or '11' for 2-circle diagram or a 
        string like '001', '010', etc, for 3-circle diagram.
        Alternatively, if the string 'A', 'B'  (or 'C' for 3-circle diagram) is given, the label of the
        corresponding set is returned (or None).'''
        if len(id) == 1:
            return self.set_labels[self.id2idx[id]] if self.set_labels is not None else None
        else:
            return self.subset_labels[self.id2idx[id]]
Returns a subset label by a "region id". 
        A region id is a string '10', '01' or '11' for 2-circle diagram or a 
        string like '001', '010', etc, for 3-circle diagram.
        Alternatively, if the string 'A', 'B'  (or 'C' for 3-circle diagram) is given, the label of the
        corresponding set is returned (or None).