[xsl] Counting preceding nodes

["Billie" <whynot77@xxxxxxxxxxxxx>]

I need help with a problem I'm having.  I need to count the number of nodes that come before the 
given node, anywhere in the tree.

So for example, say I have this tree:

<book>
    <chapter>
        <para>
            Blah blah blah
            <diagram name="first"/>
        </para>
        <para>Blah blah blah</para>
        <para>
            <diagram name="second"/>
            Blah blah blah
        </para>
        <diagram name="third"/>
    </chapter>
    <chapter>
        <para>Blah blah blah</para>
    </chapter>
    <chapter>
        <diagram name="fourth"/>
        <para>Blah blah blah</para>
    </chapter>
    <chapter>
        <para>
            <quote>
                <diagram name="fifth"/>
                <diagram name="sixth"/>
                Blah blah blah
            </quote>
        </para>
    </chapter>
</book>

I gave the diagrams the appropriate names so you could see what I was looking for, but basically, 
I'm looking for a way to number the diagrams based on the absolute order in which they are placed.  
I've tried all sorts of combinations but nothing seemed to work.  One logical structure I came up 
with, that I couldn't figure out how to translate into XSL, was: "count the number of diagrams that 
have an ancestor-or-self which is a preceding-sibling to an ancestor-or-self of the diagram in 
question".  Is that possible to do?

<xsl:template match="diagram">
    <xsl:attribute name="Number">
        <xsl:value-of select="???"/>
    </xsl:attribute>		
</xsl:template>

Thanks for your help,
Billie