code: def _set_child(self, name, child):
          
          if not isinstance(child, Parentable):
              raise ValueError('Parentable child object expected, not {child}'.format(child=child))
          child._set_parent(self)
          self._store_child(name, child)
docs: Set child.
  
          :param name: Child name.
          :param child: Parentable object.