def bottomup(cls):
        """Get all bottomup `Relationship` instances.

        Example:

            >>> from pronto import Relationship
            >>> for r in Relationship.bottomup():
            ...    print(r)
            Relationship('is_a')
            Relationship('part_of')
            Relationship('develops_from')

        """
        return tuple(unique_everseen(r for r in cls._instances.values() if r.direction=='bottomup'))