'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.'

@property
def _tr_above(self):

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