code: def pre_parse_and_validate_signavio(self, bpmn, filename):
          
          self._check_for_disconnected_boundary_events_signavio(bpmn, filename)
          self._fix_call_activities_signavio(bpmn, filename)
          return bpmn
docs: This is the Signavio specific editor hook for pre-parsing and
          validation.
  
          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)