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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 dataset(self, dataset_id, project=None):
 """Construct a reference to a dataset. :type dataset_id: str :param dataset_id: ID of the dataset. :type project: str :param project: (Optional) project ID for the dataset (defaults to the project of the client). :rtype: :class:`google.cloud.bigquery.dataset.DatasetReference` :returns: a new ``DatasetReference`` instance """

        if project is None:
            project = self.project

        return DatasetReference(project, dataset_id)