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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 save_rst(self, file_name='pysb_model.rst', module_name='pysb_module'):
 """Save the assembled model as an RST file for literate modeling. Parameters file_name : Optional[str] The name of the file to save the RST in. Default: pysb_model.rst module_name : Optional[str] The name of the python function defining the module. Default: pysb_module """

        if self.model is not None:
            with open(file_name, 'wt') as fh:
                fh.write('.. _%s:\n\n' % module_name)
                fh.write('Module\n======\n\n')
                fh.write('INDRA-assembled model\n---------------------\n\n')
                fh.write('::\n\n')
                model_str = pysb.export.export(self.model, 'pysb_flat')
                model_str = '\t' + model_str.replace('\n', '\n\t')
                fh.write(model_str)