code: def unregister(self, obj=None):
          
          if obj is None:
              self.__databases.clear()
              self.__schemas.clear()
          elif isinstance(obj, orb.Schema):
              self.__schemas.pop(obj.name(), None)
          elif isinstance(obj, orb.Database):
              if obj == self.__current_db:
                  self.__current_db = None
              self.__databases.pop(obj.name(), None)
          else:
              self.__current_db = None
              self.__schemas.pop(obj, None)
              self.__databases.pop(obj, None)
docs: Unregisters the object from the system.  If None is supplied, then
          all objects will be unregistered
  
          :param obj: <str> or <orb.Database> or <orb.Schema> or None