transform an attribute to an element

Hi,

who can tell me if I can use XQuery to transform an attribute to an element?

for example, the document is 
            <test a="123">
                 <b>321</b>
            </test>

I'd like to transform the attribute "a" to an element of test.

            <test>
               <a>123</a>
               <b>321</b>
            </test>

thank you!