public function addCell($numOfColumns = BootstrapCell::COLUMNS_NONE)
	{
		if ($this->columnsOccupied + $numOfColumns > $this->numOfColumns) {
			throw new InvalidArgumentException(
				"the given number of columns with combination of already used"
				. " columns exceeds column limit ({$this->numOfColumns})");
		}

		$cell = new BootstrapCell($this, $numOfColumns);
		$this->cells[] = $cell;

		return $cell;
	}