What I would like to do now is output a value that
appears only once throughout the XML file. For
example, if I have:

<a>
   <b>value1</b>
   <bb>value2</bb>
   <bbb>value3</bbb>
</a>
<c>
   <b>value1</b>
   <bb>value2</bb>
   <bbb>value4</bbb>
</c>