find position node controlling xpath

anyone know position node controlling xpath?

say i have following xml:

<a>
    <b>zyx</b>
    <b>wvu</b>
    <b>tsr</b>
    <b>qpo</b>
</a>

i following xpath query name third <b> node (<b>tsr</b>):

a/b[.='tsr']

which good good nonetheless i wish return ordinal position node, something like:

a/b[.='tsr']/position()

(but bit some-more working!)

is even possible?

edit: forgot plead am controlling .net 2 it's xpath 1.0!

---

update: finished adult controlling 's . those prying here's doing c#:

int position = doc.selectnodes("a/b[.='tsr']/preceding-sibling::b").count + 1;

// check node indeed exists
if (position > 1 || doc.selectsinglenode("a/b[.='tsr']") != null)
{
    console.writeline("found during position = {0}", position);
}