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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 _tr_above(self):
 """ The tr element prior in sequence to the tr this cell appears in. Raises |ValueError| if called on a cell in the top-most row. """

        tr_lst = self._tbl.tr_lst
        tr_idx = tr_lst.index(self._tr)
        if tr_idx == 0:
            raise ValueError('no tr above topmost tr')
        return tr_lst[tr_idx-1]