with-param problem

Hi,
 
I'm working on a master thesis about XSLT.
 
I found a problem and I don't know wether it is a bug in saxon (version 8.0).
 
Take a look a this little program
 
 <xsl:template match="/a">
  <xsl:element name="S"> 
   <xsl:call-template name="f">
    <xsl:with-param name="p1" select="3"/>
    <xsl:with-param name="p2" select="$p1 + 2"/>
   </xsl:call-template>
  </xsl:element>
 </xsl:template>
 
 <xsl:template name="f">
  <xsl:param name="p1"/>
  <xsl:param name="p2"/>
 </xsl:template>
 
The problem is the param named "p2".  It uses param "p1".  Why is that not
allowed in XSLT? Or is it maybe a bug in Saxon?
 
I'm sorry for my terrible English!
 
greetings
 
Wim Janssen