Double muenchian grouping...

I have an xml file which i use to generate PDF document:
<?xml version="1.0"?>
<library>
   <!--math books-->
   <book>
       <writers>Henry VANDEN, Christina MONAY, Jarod SITH</writers>
       <title>The National Council of Teachers of Mathematics:
Teaching of Arithmetic (10th yearbook)</title>
       <year>2002a</year>
       <publisher>Hermes</publisher>
       <pages>231</pages>
       <field>math</field>
   </book>
   <book>
       <writers>Ronald Staszkow</writers>
       <title>Developmental Mathematics: Basic Arithmetic with
Introductions to Algebra and Geometry</title>
       <year>2002b</year>
       <publisher>Danaroom</publisher>
       <pages>156</pages>
       <field>math</field>
   </book>
   <book>
       <writers>B.S.S, D.M.</writers>
       <title>Basic Arithmetic</title>
       <year>2002b 2002a</year>
       <publisher>Zebruje</publisher>
       <pages>120</pages>
       <field>math</field>
   </book>
   <!--history books-->
   <book>
       <writers>Boray Dim, Jessica FOLGHenry VANDEN, Christina MONAY,
Jarod SITH</writers>
       <title>History of europe</title>
       <year>2002a</year>
       <publisher>Gouray</publisher>
       <pages></pages>
       <field>history</field>
   </book>
   <book>
       <writers>A.C., B.D.</writers>
       <title>history example 1</title>
       <year>2002a 2002b</year>
       <publisher>Hermes</publisher>
       <pages></pages>
       <field>history</field>
   </book>
   <book>
       <writers>corray Stakovic</writers>
       <title>The 1st world war 1914-1918</title>
       <year>2002a</year>
       <publisher>Hermes</publisher>
       <pages></pages>
       <field>history</field>
   </book>
</library>

First i liked to group using 'field' and it works:

<?xml version="1.0"?>
<xsl:stylesheet version="1.1" xmlns:xsl="
http://www.w3.org/1999/XSL/Transform"
xmlns:fo="http://www.w3.org/1999/XSL/Format"
exclude-result-prefixes="fo">
<!--Using Muenchian Grouping-->
<xsl:key name="byField" match="book" use="field" />
<xsl:output method="xml" version="1.0" omit-xml-declaration="no" indent="yes"/>

   <xsl:template match="library">
       <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format ">
           <fo:layout-master-set>
               <fo:simple-page-master master-name="simpleA4"
page-height="29.7cm" page-width="21cm" margin-top="2cm"
margin-bottom="2cm" margin-left="2cm" margin-right="2cm">
                   <fo:region-body />
               </fo:simple-page-master>
           </fo:layout-master-set>
           <fo:page-sequence master-reference="simpleA4"
initial-page-number="1">

               <fo:flow flow-name="xsl-region-body">
                   <fo:block text-align="center" wrap-option="wrap"
font-size="10pt" font-weight="bold" space-after="5mm">
                       <fo:inline text-decoration="underline">
                           <xsl:text>Publication list :</xsl:text>
                       </fo:inline>
                   </fo:block>
                   <fo:block font-size="10pt">
                       <xsl:apply-templates
select="book[generate-id(.) = generate-id(key('byField', field)[1])]"
/>
                   </fo:block>
               </fo:flow>
           </fo:page-sequence>
       </fo:root>
   </xsl:template>


   <xsl:template match="book">
       <fo:block space-before=" 0.25">

               <xsl:choose>
                   <xsl:when test="field = 'math'">
                       <fo:inline text-decoration="underline">
                           <xsl:text>Mathematics :</xsl:text>
                       </fo:inline>

                       <xsl:for-each select="key('byField', field)">
                           <fo:block space-before="0.25cm"
space-after="0.25cm" intrusion-displace="line" text-align="justify">
                               <xsl:text>-&#171;</xsl:text>
                               <xsl:value-of select="title"/>
                               <xsl:text>&#187;, </xsl:text>

                               <xsl:value-of select="writers"/>
                               <xsl:text>. Publisher : </xsl:text>

                               <xsl:value-of select="publisher"/>
                               <xsl:text>. Pages : </xsl:text>

                               <xsl:value-of select="publisher"/>
                               <xsl:text>.</xsl:text>
                           </fo:block>
                       </xsl:for-each>
                   </xsl:when>

                   <xsl:when test="field = 'history'">
                       <fo:inline text-decoration="underline">
                           <xsl:text>Hitory :</xsl:text>
                       </fo:inline>

                       <xsl:for-each select="key('byField', field)">
                           <fo:block space-before="0.25cm"
space-after="0.25cm" intrusion-displace="line" text-align="justify">
                               <xsl:text>-</xsl:text>
                               <xsl:value-of select="writers"/>
                               <xsl:text>, </xsl:text>

                               <xsl:text>&#171;</xsl:text>
                               <xsl:value-of select="title"/>
                               <xsl:text>&#187;. Publisher : </xsl:text>

                               <xsl:value-of select="publisher"/>
                               <xsl:text>.</xsl:text>
                           </fo:block>
                       </xsl:for-each>
                   </xsl:when>

                   <xsl:otherwise>
                       <fo:inline text-decoration="underline">
                           <xsl:text>Others:</xsl:text>
                       </fo:inline>

                       <xsl:for-each select="key('byField', field)">
                           <fo:block space-before="0.25cm"
space-after=" 0.25cm " intrusion-displace="line" text-align="justify">
                               <xsl:text>-</xsl:text>
                               <xsl:value-of select="writers"/>
                               <xsl:text>.</xsl:text>

                               <xsl:text>&#171;</xsl:text>
                               <xsl:value-of select="title"/>
                               <xsl:text>&#187;, </xsl:text>
                           </fo:block>
                       </xsl:for-each>
                   </xsl:otherwise>

               </xsl:choose>
       </fo:block>
   </xsl:template>
</xsl:stylesheet>

It gives me something like this:

Publication list :
Mathematics :
.....
.....
History :
.....
.....
Others :
.....
.....

Now i want to add grouping using 'year' , i want to get  something like:

Publication list :
2002a:
Mathematics :
.....
.....
History :
.....
.....
Others :
.....
.....

2002b:
Mathematics :
.....
.....
History :
.....
.....
Others :
.....
.....

Which mean using double grouping....
I tried to do the same thing but i doesn't work... and it's more complicated...
perhaps there's another way to do that..
Please can someone help me...