code: def _drop_cascade_relation(self, dropped):
          
          if dropped not in self.relations:
              logger.debug('dropped a nonexistent relationship: {!s}'
                           .format(dropped))
              return
          consequences = self.relations[dropped].collect_consequences()
          logger.debug(
              'drop {} is cascading to {}'.format(dropped, consequences)
          )
          self._remove_refs(consequences)
docs: Drop the given relation and cascade it appropriately to all
          dependent relations.
  
          :param _CachedRelation dropped: An existing _CachedRelation to drop.