[xpath] Find position of a node using xpath

You can do this with XSLT but I'm not sure about straight XPath.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" encoding="utf-8" indent="yes" 
              omit-xml-declaration="yes"/>
  <xsl:template match="a/*[text()='tsr']">
    <xsl:number value-of="position()"/>
  </xsl:template>
  <xsl:template match="text()"/>
</xsl:stylesheet>