public function rowIndex($position = null)
    {
        if ($position === null) {
            $position = $this->position();
        }

        $i = 0;
        $p = 0;
        foreach ($this->structure as $row_ident => $row) {
            $numCells = count($row['columns']);
            $p += $numCells;
            if ($p > $position) {
                return $i;
            }
            $i++;
        }
        return null;
    }