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

        Returns:
            :obj:`generator`

        Example:

            >>> from pronto import Relationship
            >>> for r in Relationship.topdown():
            ...    print(r)
            Relationship('can_be')
            Relationship('has_part')

        """
        return tuple(unique_everseen(r for r in cls._instances.values() if r.direction=='topdown'))
Get all topdown `Relationship` instances.

        Returns:
            :obj:`generator`

        Example:

            >>> from pronto import Relationship
            >>> for r in Relationship.topdown():
            ...    print(r)
            Relationship('can_be')
            Relationship('has_part')