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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 previousSibling(self) -> Optional[AbstractNode]: """Return the previous sibling of this node. If there is no previous sibling, return ``None``. """

        parent = self.parentNode
        if parent is None:
            return None
        return parent.childNodes.item(parent.childNodes.index(self) - 1)