Grouping subsequent like elements with XSLT

[admin]

Hi, I have the following XML:

<test>
<a>1</a>
<a>2</a>
<b>3</b>
<a>4</a>
<a>5</a>
<b>6</b>
<b>7</b>
<b>8</b>
<a>9</a>
<b>10</b>
<a>11</a>
</test>

There can be any number of <a> or <b> elements in any order. I would like to be able to group by subsequent like elements as follows using XSLT:

<test>
<group>
<a>1</a>
<a>2</a>
</group>
<group>
<b>3</b>
</group>
<group>
<a>4</a>
<a>5</a>
</group>
<group>
<b>6</b>
<b>7</b>
<b>8</b>
</group>
<group>
<a>9</a>
</group>
<group>
<b>10</b>
</group>
<group>
<a>11</a>
</group>
</test>

I think this may be an interesting little problem (only because I cannot solve it). Any help greatly appreciated.