code: def topdown(cls):
          
          return tuple(unique_everseen(r for r in cls._instances.values() if r.direction=='topdown'))
docs: 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')