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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 from_string(cls, model_id, default_project=None):
 """Construct a model reference from model ID string. Args: model_id (str):
 A model ID in standard SQL format. If ``default_project`` is not specified, this must included a project ID, dataset ID, and model ID, each separated by ``.``. default_project (str):
 Optional. The project ID to use when ``model_id`` does not include a project ID. Returns: google.cloud.bigquery.model.ModelReference: Model reference parsed from ``model_id``. Raises: ValueError: If ``model_id`` is not a fully-qualified table ID in standard SQL format. """

        proj, dset, model = _helpers._parse_3_part_id(
            model_id, default_project=default_project, property_name="model_id"
        )
        return cls.from_api_repr(
            {"projectId": proj, "datasetId": dset, "modelId": model}
        )