Problem while customizing title's procedure

Hello,

Using docbook-xsl 1.69.1 and fop 0.25, I've tried to get a different  
spacing after the title for procedure (title is before procedure)  
while using the same values for other formal title properties'  
parameters, as following:

<xsl:attribute-set name="formal.title.properties"
                    use-attribute-sets="normal.para.spacing">
   <xsl:attribute name="font-weight">bold</xsl:attribute>
   <xsl:attribute name="font-size">10pt</xsl:attribute>
   <xsl:attribute name="hyphenate">false</xsl:attribute>
   <xsl:attribute name="space-after.minimum">
     <xsl:choose>
       <xsl:when test="self::procedure">0em</xsl:when>
       <xsl:otherwise>inherit</xsl:otherwise>
     </xsl:choose>
   </xsl:attribute>
   <xsl:attribute name="space-after.optimum">
     <xsl:choose>
       <xsl:when test="self::procedure">0em</xsl:when>
       <xsl:otherwise>inherit</xsl:otherwise>
     </xsl:choose>
   </xsl:attribute>
   <xsl:attribute name="space-after.maximum">
     <xsl:choose>
       <xsl:when test="self::procedure">0em</xsl:when>
       <xsl:otherwise>inherit</xsl:otherwise>
     </xsl:choose>
   </xsl:attribute>
</xsl:attribute-set>

The pdf is rendered correctly, but xsltproc issues a warning:
xsl:attribute-set : use-attribute-sets recursion detected

Am I completely wrong doing it this way?

Thanks in advance for your help,

Mich鋩e