code: def update_message(self, message_id, category_id, title, body,
          extended_body, use_textile=False, private=False, notify=None):
          
          path = '/msg/update/%u' % message_id
          req = ET.Element('request')
          req.append(self._create_message_post_elem(category_id, title, body,
              extended_body, use_textile=False, private=False))
          if notify is not None:
              for person_id in notify:
                  ET.SubElement(req, 'notify').text = str(int(person_id))
          return self._request(path, req)
docs: Updates an existing message, optionally sending notifications to a
          selected list of people. Note that you can also upload files using
          this function, but you have to format the request as
          multipart/form-data. (See the ruby Basecamp API wrapper for an example
          of how to do this.)