'Return None.  Insert a row of headers,
where ``headers`` is a sequence of strings.
(The strings may contain newlines, to indicated multiline headers.)'

def insert_header_row(self, rownum, headers, dec_below='header_dec_below'):

    header_rows = [header.split('\n') for header in headers]
    rows = list(zip_longest(*header_rows, **dict(fillvalue='')))
    rows.reverse()
    for (i, row) in enumerate(rows):
        self.insert(rownum, row, datatype='header')
        if (i == 0):
            self[rownum].dec_below = dec_below
        else:
            self[rownum].dec_below = None
