def select(self, cond, multiple=False):
raise NotImplementedError<EOL>
Args:
    cond (:obj:`tuple`): query expressiom
    multiple (:obj:`bool`): whether or not to select the multiple elements. If set to True, the method
                            terminates immediately once the node is found, otherwise it traverses through all
                            nodes and then exists

Returns:
    :obj:`list`: list <inherited from :py:class:`AbstractNode <poco.sdk.AbstractNode>`>