Is it possible to know position of ancestor?

Let say we have such XML:
<root>
        <line>
                <a/>
                <b/>
        </line>
        <line>
                <a/>
                <b/> - (I?m here)
        </line>
??
</root> 

We are in tag "b", which in second tag "line". But we don't know what we are 
in second tag "line". We want to find out what is position of our ancestor 
in their ancestor.
Is it possible using Xpath? 

Thanks,
Jenya 

XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list