[xsl] How to test with XPath the existence of a certain branch of a <choice> statement?

["Ben Stover" <bxstover@xxxxxxxxxxx>]

For an XSLT script I need to check wether one certain branch of a <choice> statement is present.
The critical part of the XML statement look like as follows (<choice> and <sequence> are taken/inserted from the 
underlying XSD schema file):

<parent>
  <choice>
     <sequence>
        <aaa>...</aaa>
        <bbb>...</bbb>
    </sequence>
      <sequence>
        <aaa>...</aaa>
        <ccc>...</ccc>
    </sequence>
     <sequence>
        .....
    </sequence>
  </choice>
</parent>

As you can see a XML doc can contain one of the three sequence branches.

How do I check with XPath if the first branch is currently filled in an XML doc?

If I code e.g.

  test=.../parent/aaa

then it is not clear if the first or second branch is present.

Can anyone give me a hint?

Thank you
Ben