function setMerge($first_row, $first_col, $last_row, $last_col)
    {
        if (($last_row < $first_row) || ($last_col < $first_col)) {
            return;
        }
        // don't check rowmin, rowmax, etc... because we don't know when this
        // is going to be called
        $this->_merged_ranges[] = array($first_row, $first_col, $last_row, $last_col);
    }