[xsl] Subdividing sets of elements by sequence identifiers

["Mark" <mark@xxxxxxxxxxxx>]

I have an "almost works" situation. The template and simplified input are 
below. What I want to is "re-wrap images with consecutive image numbers (in 
my output example "27,28,29") in a new <h6>  element for HTML display 
purposes. To do this,at the beginning of such a sequence, I must close off 
the <h6> inserted by the current template with a </h6> and open a new <h6>. 
Here only one sequence is shown, but several are possible within each 
element.

I suspect I am thinking about this procedurally rather than imperatively, 
and thus cannot see the forest for the trees. All suggestions appreciated.
Mark

Selection from current output:
<List>
 <h1>Art on Stamps</h1>
 <h2>1993</h2>
 <h6>
   <a href="../img/1993/5.jpg" target="_blank">
     <img src="../img/1993/5.jpg" alt="stamp image" width="100" 
border="2px"></img>
   </a>
<!-- ********** need to insert </h6><h6> here ************* -->

   <a href="../img/1993/27.jpg" target="_blank">
     <img src="../img/1993/27.jpg" alt="stamp image" width="100" 
border="2px"></img>
   </a>
   <a href="../img/1993/28.jpg" target="_blank">
     <img src="../img/1993/28.jpg" alt="stamp image" width="100" 
border="2px"></img>
   </a>
   <a href="../img/1993/29.jpg" target="_blank">
     <img src="../img/1993/29.jpg" alt="stamp image" width="100" 
border="2px"></img>
   </a>
 </h6>
......
</List>

My Template:
<xsl:template match="Item">
  <h1>
    <xsl:value-of select="@concept"></xsl:value-of>
  </h1>
  <xsl:for-each-group select="Stamp" group-by="Date/@year">
    <xsl:sort select="Date/@year"></xsl:sort>
    <h2>
      <xsl:value-of select="Date/@year"></xsl:value-of>
    </h2>
    <h6>
      <xsl:for-each select="current-group()">
        <xsl:variable name="image" select="concat('../img/',  Date/@year, 
'/', @image-number, '.jpg')"></xsl:variable>
        <a href="{$image}" target="_blank">
          <img src="{$image}" alt="stamp image" width="100" 
border="2px"></img>
        </a>
      </xsl:for-each>
    </h6>
  </xsl:for-each-group>
</xsl:template>

My Input: (simplified to a single <Item> from the <List>
<List>
 <Item concept="Art on Stamps">
   <Stamp image-number="5">
     <Date year="1993"></Date>
   </Stamp>
   <Stamp image-number="27" >
     <Date year="1993"></Date>
   </Stamp>
   <Stamp image-number="28">
     <Date year="1993"></Date>
   </Stamp>
   <Stamp image-number="29" >
     <Date year="1993"></Date>
   </Stamp>
   <Stamp image-number="42" >
     <Date year="1994"></Date>
   </Stamp>
   <Stamp image-number="43">
     <Date year="1994"></Date>
   </Stamp>
   <Stamp image-number="44">
     <Date year="1994"></Date>
   </Stamp>
   <Stamp image-number="57" >
     <Date year="1994" ></Date>
   </Stamp>
   <Stamp image-number="58">
     <Date year="1994"></Date>
   </Stamp>
   <Stamp image-number="59">
     <Date year="1994"></Date>
   </Stamp>
   <Stamp image-number="96">
     <Date year="1995"></Date>
   </Stamp>
   <Stamp image-number="97" >
     <Date year="1995"></Date>
   </Stamp>
   <Stamp image-number="98">
     <Date year="1995" ></Date>
   </Stamp>
 </Item>
</List>