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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 columns_used(self):
 """ Returns all the columns used across all models in the group for filtering and in the model expression. """

        return list(tz.unique(tz.concatv(
            util.columns_in_filters(self.fit_filters),
            util.columns_in_filters(self.predict_filters),
            util.columns_in_formula(self.default_model_expr),
            self._group.columns_used(),
            [self.segmentation_col])))