partial selection in tree view with checkbox

<zk xmlns:w="client">
	<style>
		.mixed-state {
			margin: 0px;
			border: 2px solid #3E48AE;
		}
	</style>
	<script type="text/javascript"><![CDATA[
		zk.afterLoad("zul.sel", function () {
		var oldSelect = zul.sel.Treeitem.prototype.doSelect_;
			zul.sel.Treeitem.prototype.doSelect_ = function (evt) {
				oldSelect.apply(this, arguments);
 
				var $dom = jq(evt.domTarget),
					row = $dom.hasClass('z-treerow')? $dom[0] : $dom.parents('.z-treerow')[0],
					isSelected = jq(row).hasClass('z-treerow-seld'),
					isSelfSelected = jq(this.treerow.$n()).hasClass('z-treerow-seld'),
					_selectedChild = this._selectedChild,
					index, isRecord;
 
				if (!_selectedChild)
					_selectedChild = this._selectedChild = [];
 
				index = _selectedChild.indexOf(row);
				isRecord = index > -1;
				if (isSelected && !isRecord)
					_selectedChild.push(row);
				else if (!isSelected && isRecord)
					_selectedChild.splice(index, 1);
 
				if (_selectedChild.length > 0 && !isSelfSelected)
					jq(this.treerow.$n()).find('.z-treerow-img-checkbox').addClass('mixed-state');
				else
					jq(this.treerow.$n()).find('.z-treerow-img-checkbox').removeClass('mixed-state');
			}
		});
	]]></script>
	<tree id="tree" rows="5" multiple="true" checkmark="true" height="400px">
		<treecols>
			<treecol label="Name"/>
			<treecol label="Description"/>
		</treecols>
		<treechildren>
			<treeitem>
				<treerow>
					<treecell label="Item 1"/>
					<treecell label="Item 1 description"/>
				</treerow>
			</treeitem>
			<treeitem>
				<treerow>
					<treecell label="Item 2"/>
					<treecell label="Item 2 description"/>
				</treerow>
				<treechildren>
					<treeitem>
						<treerow>
							<treecell label="Item 2.1"/>
						</treerow>
						<treechildren>
							<treeitem>
								<treerow>
									<treecell label="Item 2.1.1"/>
								</treerow>
							</treeitem>
							<treeitem>
								<treerow>
									<treecell label="Item 2.1.2"/>
								</treerow>
							</treeitem>
						</treechildren>
					</treeitem>
					<treeitem>
						<treerow>
							<treecell label="Item 2.2"/>
							<treecell label="Item 2.2 is something who cares"/>
						</treerow>
					</treeitem>
				</treechildren>
			</treeitem>
			<treeitem label="Item 3"/>
		</treechildren>
	</tree>
</zk>