docs: def add_droppable(self, droppable_text, name='', reuse=1, drop_behavior_type=None):
          """stub"""
          if not isinstance(droppable_text, DisplayText):
              raise InvalidArgument('droppable_text is not a DisplayText object')
          if not isinstance(reuse, int):
              raise InvalidArgument('reuse must be an integer')
          if reuse < 0:
              raise InvalidArgument('reuse must be >= 0')
          if not isinstance(name, DisplayText):
              # if default ''
              name = self._str_display_text(name)
          droppable = {
              'id': str(ObjectId()),
              'texts': [self._dict_display_text(droppable_text)],
              'names': [self._dict_display_text(name)],
              'reuse': reuse,
              'dropBehaviorType': drop_behavior_type
          }
          self.my_osid_object_form._my_map['droppables'].append(droppable)
          return droppable