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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 create_bmi_config_file(self, filename: str = "bmi_config.txt") -> None: """ Create a BMI config file to initialize the model. Args: filename: The filename with which the config file should be saved. """

        s0 = self.construct_default_initial_state()
        s0.to_csv(filename, index_label="variable")