code: def print_model(self, pretty=True, encoding='utf8'):
          
          return lxml.etree.tostring(self.sbgn, pretty_print=pretty,
                                     encoding=encoding, xml_declaration=True)
docs: Return the assembled SBGN model as an XML string.
  
          Parameters
          ----------
          pretty : Optional[bool]
              If True, the SBGN string is formatted with indentation (for human
              viewing) otherwise no indentation is used. Default: True
  
          Returns
          -------
          sbgn_str : bytes (str in Python 2)
              An XML string representation of the SBGN model.