Driving me crazy:- Why can't I use the standard W3C XPath functions with XslTransform ?

I have this as one of the namespaces

                xmlns:fn="http://www.w3.org/2005/xpath-functions"

I am using a string xpath function like this

      <xsl:value-of  select="fn:substring-after(currentnode, 'First')"/>
 

I got the following error when I try to transform using MS Xml Libraries ? Why in the world Microsoft would do that ? Are they trying to force us into using msxml ? Why would they ditch the standard XPath functions ?

An unhandled exception of type 'System.Xml.Xsl.XsltException' occurred in System.Data.SqlXml.dll

Additional information: Cannot find the script or external object that implements prefix 'fn'.

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Fahad