<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def save_model(self, file_name=None):
 """Save the BioPAX model object in an OWL file. Parameters file_name : Optional[str] The name of the OWL file to save the model in. """

        if file_name is None:
            logger.error('Missing file name')
            return
        pcc.model_to_owl(self.model, file_name)