'Return None. Adds row-instance specific formatting
for the specified output format.
Example: myrow.add_format(\'txt\', row_dec_below=\'+-\')'

def add_format(self, output_format, **fmt_dict):

    output_format = get_output_format(output_format)
    if (output_format not in self.special_fmts):
        self.special_fmts[output_format] = dict()
    self.special_fmts[output_format].update(fmt_dict)
