code: def manage(cls, entity, unit_of_work):
          
          if hasattr(entity, '__everest__'):
              if not unit_of_work is entity.__everest__.unit_of_work:
                  raise ValueError('Trying to register an entity that has been '
                                   'registered with another session!')
          else:
              entity.__everest__ = cls(entity, unit_of_work)
docs: Manages the given entity under the given Unit Of Work.
  
          If `entity` is already managed by the given Unit Of Work, nothing
          is done.
  
          :raises ValueError: If the given entity is already under management
            by a different Unit Of Work.