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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 _prior_headerfooter(self):
 """|_Footer| proxy on prior sectPr element or None if this is first section."""

        preceding_sectPr = self._sectPr.preceding_sectPr
        return (
            None if preceding_sectPr is None
            else _Footer(preceding_sectPr, self._document_part, self._hdrftr_index)
        )