code: def write_model_to_file(self, mdl, fn):
          
          # Write the model to file
          output = ''.join(self.post_process_model(mdl, self.add_line_refs))
          if fn:
              fqfn = self.dst_dir + '/' + fn
              if os.path.isfile(fqfn):
                  self.error("File '%s' exists" % fqfn)
                  return
              with open(fqfn, 'w') as of:
                  of.write(output)
                  of.close()
                  self.extracted_models.append(fn)
          else:
              self.error("Output file name can not be determined; YANG file not created")
docs: Write a YANG model that was extracted from a source identifier
          (URL or source .txt file) to a .yang destination file
          :param mdl: YANG model, as a list of lines
          :param fn: Name of the YANG model file
          :return: