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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_below(self):
 """ The tr element next in sequence after the tr this cell appears in, or |None| if this cell appears in the last row. """

        tr_lst = self._tbl.tr_lst
        tr_idx = tr_lst.index(self._tr)
        try:
            return tr_lst[tr_idx+1]
        except IndexError:
            return None