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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 data_children(self) -> List["DataNode"]: """Return the set of all data nodes directly under the receiver."""

        res = []
        for child in self.children:
            if isinstance(child, DataNode):
                res.append(child)
            elif not isinstance(child, SchemaTreeNode):
                res.extend(child.data_children())
        return res