Maths functions in a stylesheet?

If any XSLT gurus could help with  this...

I have a "maths functions" stylesheet, adapted from another source -

<?xml version="1.0"?> 
<xsl:stylesheet 
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
 xmlns:xalan="http://xml.apache.org/xalan";
 xmlns:maths="http://www.mydomain.com/maths";
 extension-element-prefixes="maths"
 version="1.0">
<xalan:component prefix="maths" elements="rules" functions="sine cosine
abs pi">
 <xalan:script lang="javascript">         
   function sine(thisValue) {
     return Math.sin(thisValue);
   }
   function cosine(thisValue) {
     return Math.cos(thisValue);
   }
   function abs(thisValue) {
     return Math.abs(thisValue);
   }   
   function pi() {
     return 3.141592653589790;
   }                             
 </xalan:script>
</xalan:component>
 </xsl:stylesheet>

Called like this:

<?xml version="1.0"?>
<xsl:stylesheet 
 version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
 xmlns:xalan="http://xml.apache.org/xalan";
 xmlns:maths="http://www.mydomain.com/maths";
 extension-element-prefixes="maths"> 
 >
<xsl:include href="functions.xsl"/>  
<xsl:template match="/">
 <xsl:variable name="foo" select="maths:pi()" />
</xsl:template>
</xsl:stylesheet>

I get :

An Error Occurred
java.lang.RuntimeException: Unknown error in XPath.
cause: java.lang.RuntimeException: Unknown error in XPath.

when running this with Cocoon 2.1.5 (withe default build with Xalan).

Thanks
Derek