multiple range expansion

Dear All,

I am having the following inputs:

1. <cross-ref>[<cr refid="MEP_L_bib5">5</cr>-<cr
refid="MEP_L_bib7">7</cr>, <cr refid="MEP_L_bib21">21</cr>-<cr
refid="MEP_L_bib24">24</cr>]</cross-ref>

2. <cross-ref>[<cr refid="MEP_L_bib2">2</cr>,<cr
refid="MEP_L_bib4">4</cr>]</cross-ref>

3. <cross-ref>[<cr refid="MEP_L_bib5">5</cr>-<cr
refid="MEP_L_bib7">7</cr>]</cross-ref>

the required output is as follows:

1. <cross-refs refid="bib5 bib6 bib7 bib21 bib22 bib23 bib24">[5-7,
21-24]</cross-refs>

2. <cross-refs refid="bib2 bib4">[2,  4]</cross-refs>

3.  <cross-refs refid="bib5 bib6 bib7">[5-7]</cross-refs>

I have tried this with the following code:

<xsl:template match="cross-ref">
<xsl:choose>
<xsl:when test="contains(.,'-')">
   <ce:cross-refs>
     <xsl:attribute name="refid">
         <xsl:if test="cr/@refid[contains(., 'bib')]">
	      <xsl:variable name="start"
select="cr[following-sibling::text()[1] =
'-']/@refid/xs:integer(substring-after(.,'MEP_L_bib'))"/>
	      <xsl:variable name="end"
select="cr[preceding-sibling::text()[1] =
'-']/@refid/xs:integer(substring-after(.,'MEP_L_bib'))"/>
	      <xsl:value-of select="for $i in $start to $end return
concat('bib', $i)"/>
	</xsl:if>
    </xsl:attribute>
    <xsl:apply-templates/>
   </ce:cross-refs>
 </xsl:when>
 <xsl:otherwise>
     <ce:cross-refs>
	<xsl:attribute name="refid">
	   <xsl:value-of select="substring-after(cr[1]/@refid,'MEP_L_')"/>
	     <xsl:for-each select="cr[position() != 1]">
		 <xsl:text> </xsl:text>
		 <xsl:value-of select="substring-after(@refid,'MEP_L_')"/>
	      </xsl:for-each>
	</xsl:attribute>
	<xsl:apply-templates/>
     </ce:cross-refs>
  </xsl:otherwise>
</xsl:choose>
</xsl:template>


When I execute the above code i am getting the following error:

  XPTY0004: A sequence of more than one item is not allowed as the
first operand of 'to' (5, 21)

I have tried with

<xsl:for-each select="$start and $end">
     <xsl:value-of select="for $i in $start to $end return concat('bib', $i)"/>
</xsl:for-each>

Even this also not working. Please help me in the solving the above problem.

Regards,
Ganesh