sax getLineNumber under saxon

I'm trying to output the line number of an 'error'
(according to schematrons definition) in the XML 
source file (with a weak comprehension of java<grin/>)

I think I'm nearly there with the following

<?xml version="1.0"?>

<xsl:stylesheet
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  
    
    version="1.0">
  <xsl:output type ='text'/>


  <xsl:template match="/doc/section/p"> **************************
<xsl:variable name="lineno" select="sax:getLineNumber()"
  xmlns:sax="/org.xml.sax.Locator"
/> 
  <xsl:value-of select="$lineno"/>
  </xsl:template>

</xsl:stylesheet>

Saxon tells me


 Error processing source document
At xsl:variable on line 21 of file:/f:/sgml/xsl/trash.xsl: Cannot
instantiate 
object for extension function getLineNumber
Elapsed time: 1462 milliseconds


My interpretation of this is nil.
Could anyone help please.

I understand that the line number will be
'later' than the actual, but close enough (I hope).

On the wish list,
given

file root.xml

<doc>

&chap1;

</doc>

etc, I.e. documents which include others,
is Sax clever enough to tell me which file its
looking at as well?

TIA, DaveP


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