如何在div中包装一组div

I have this html code:

我有这个HTML代码:

<div id="elem">
    <div data-foo="aaa">a</div>
    <div data-foo="aaa">a</div>
    <div data-foo="aaa">a</div>
    <div data-foo="bbb">b</div>
    <div data-foo="bbb">b</div>
    <div data-foo="ccc">c</div>
    <div data-foo="ccc">c</div>
    <div data-foo="ccc">c</div>
    <div data-foo="ccc">c</div>
    <div data-foo="ccc">c</div>
</div>
<div id=