code: def pre_parse_and_validate(self, bpmn, filename):
          
          bpmn = self._call_editor_hook(
              'pre_parse_and_validate', bpmn, filename) or bpmn
  
          return bpmn
docs: A subclass can override this method to provide additional parseing or
          validation. It should call the parent method first.
  
          :param bpmn: an lxml tree of the bpmn content
  
          :param filename: the source file name
  
          This must return the updated bpmn object (or a replacement)