Failing string() test fail

What would cause a string() test to fail?  I've tested on a simple
xml, like this, with the following test:

Assume we are matching on B

<xsl:if test="string(C/@test)">Yes</xsl:if> You get one Yes.

<A>
    <B>
        <C/>
        <C/>
        <C/>
    </B>
    <B>
        <C/>
        <C test="yes"/>
        <C/>
    </B>
</A>


Of course I have a much more complicated XML, but the premise is the
same.  So isn't the above test valid?  If the test was 3 deep and the
context is A, so: B/C/@test, isn't that also valid?

Karl..

Karl..