Optgoup in combobox

<zk xmlns:h="http://www.w3.org/1999/xhtml">
			<h:select>
				<h:optgroup label="Swedish Cars">
					<h:option value="volvo">Volvo</h:option>
					<h:option value="saab">Saab</h:option>
				</h:optgroup>
				<h:optgroup label="German Cars">
					<h:option value="mercedes">Mercedes</h:option>
					<h:option value="audi">Audi</h:option>
				</h:optgroup>
			</h:select>
		</zk>