[Saxon] <xsl:choose> inside <xsl:attribute-set>

First, I would like to thank Nikolai Grigoriev and Sebastian Rahtz for
their very helpful answers to my question about using CSS to emulate FO.
I am now woking at outputting CSS enriched HTML as Sebastian suggested.

I have a problem wich looks very much like a bug in Saxon.
I am using Saxon 5.4.1.

- - - toto.xsl - - - 
<?xml version="1.0" encoding="iso-8859-1"?> 
<xsl:stylesheet version="1.0"
		xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:attribute-set name="foo">
    <xsl:attribute name="bar">
      <xsl:choose>
	<xsl:when test="/toto">Toto</xsl:when>
	<xsl:otherwise>Not Toto</xsl:otherwise>
      </xsl:choose>
    </xsl:attribute>
  </xsl:attribute-set>

  <xsl:template match="/">
    <toto xsl:use-attribute-set="foo"/>
  </xsl:template>
    
</xsl:stylesheet>
- - - - - - - - - -

$ java com.icl.saxon.StyleSheet toto.xml toto.xsl
Failed to compile style sheet
At xsl:choose on line 8 of file:/home/david/tmp/toto.xsl: Element must
only be used within a template

- - - XSLT Programmer's Reference (p. 164) - - -
An attribute set in not simply a textual macro. The attributes contained
in the attribute set each have a template body to define the value, and
although this will often be a simple text node, it may also, for example,
declare variables or invoke other XSLT instructions such as
<xsl:call-template> and <xsl:apply-templates>.
- - - - - - - - - - - - - - - - - - - - - - - - -

Is there actually a contradiction, or a I am misunderstanding something ?

Actually, I can work around the problem by using named templates instead
of attribute sets, but using attribute sets gives a useful clue to the
human reader (I am working on an semi-automated conversion tool).  

Moreover this upsets me as a wrongness in my grokking of XSLT.

                             -- David --

PS: Kay's book is the One True Book (tm) on XSLT



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