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>