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.

def manage(cls, entity, unit_of_work):
        """
        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.
        """
        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)