def add_segment(self, name, model_expression=None):
        """
        Add a new segment with its own model expression.

        Parameters
        ----------
        name
            Segment name. Must match a segment in the groupby of the data.
        model_expression : str or dict, optional
            A patsy model expression that can be used with statsmodels.
            Should contain both the left- and right-hand sides.
            If not given the default model will be used, which must not be
            None.

        """
        logger.debug('adding LCM model {} to segmented model {}'.format(
            name, self.name))
        if not model_expression:
            if not self.default_model_expr:
                raise ValueError(
                    'No default model available, '
                    'you must supply a model expression.')
            model_expression = self.default_model_expr

        # we'll take care of some of the filtering this side before
        # segmentation
        self._group.add_model_from_params(
            name=name,
            model_expression=model_expression,
            sample_size=self.sample_size,
            probability_mode=self.probability_mode,
            choice_mode=self.choice_mode,
            choosers_fit_filters=None,
            choosers_predict_filters=None,
            alts_fit_filters=None,
            alts_predict_filters=None,
            interaction_predict_filters=self.interaction_predict_filters,
            estimation_sample_size=self.estimation_sample_size,
            choice_column=self.choice_column)
Add a new segment with its own model expression.

        Parameters
        ----------
        name
            Segment name. Must match a segment in the groupby of the data.
        model_expression : str or dict, optional
            A patsy model expression that can be used with statsmodels.
            Should contain both the left- and right-hand sides.
            If not given the default model will be used, which must not be
            None.