XSLT:Transforming a flat list in a nested one

[vv1984]

Hi to every one,
I need some help with this simple (actually not so simple for me..) tranformation.
I've got something like this:

<ul>
<li class="chapter">Chapter1</li>
<li class="sub">subchap1.1</li>
<li class="sub">subchap1.2</li>
<li class="chapter">Chapter2</li>
<li class="sub">subchap2.1</li>
<li class="sub">subchap2.2</li>
</ul>

...and I'd like to obtain this:

<select>
<optgroup label="Chapter1">
<option>subchap1.1</option>
<option>subchap1.2</option>
</optgroup>
<optgroup label="Chapter2">
<option>subchap2.1</option>
<option>subchap2.2</option>
</optgroup>
</select>

I've tried this, but does not work properly, optgroup's just
at option's same level:

<xsl:template match="ul">
<select>
<xsl:for-each select="li[@class='chapter']"> 
	
			<optgroup>
				<xsl:attribute name="label">
				<xsl:value-of select="current()"/>
				</xsl:attribute>
				
				<xsl:for-each select="li[@class='sub']">
			
					<option>
						<xsl:attribute name="value">
							<xsl:value-of select="current()"/>
						</xsl:attribute>
						<xsl:value-of select="current()"/>
					</option>	
				
				</xsl:for-each>	
			</optgroup>
	
	</xsl:for-each>
</select>
</xsl:template>


</xsl:stylesheet>

Any suggest is welcome!!

[jkmyoung]

I think it's like:
[code=xml]
<xsl:for-each select="followi ng-sibling::li[@class='sub'][generate-id(preceding-sibling::li[@class='chapter '][1]) = generate-id(current())]">
[/code]

Or you could use a key; that would require 2 lines of edits, but would be a little faster with larger files.