Function name that doesn't have an explicit namespace

Hi Folks,

Is there a way to define an xsl:function without explicitly giving it a
namespace prefix? For example:

    <xsl:function name="f">

    </xsl:function>

I was thinking that using xpath-default-namespace would enable me to do what I
desire:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                         xpath-default-namespace="blah"
                         version="3.0">

    <xsl:function name="f">
          ...
    </xsl:function>

But I get the error:

    Function name must have a namespace prefix

Suggestions?

/Roger