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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def expr(self, callback):
 """Filter by custom expression. :param callback: Callback for custom expression. :type name: ``function`` :return: A list of Node objects. :rtype: ``list`` of :class:`Node` """

        nodes = []
        for node in self.nodes:
            if callback(node):
                nodes.append(node)
        self.nodes = nodes
        return self