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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 _get_node(self, treelist, pos):
 """ look up widget at `pos` of `treelist`; default to None if nonexistent. """

        node = None
        if pos is not None:
            subtree = self._get_substructure(treelist, pos)
            if subtree is not None:
                node = subtree[0]
        return node