[xsl] Multiple matches against keys?

["Ramon M. Felciano @ Yahoo" <felciano@xxxxxxxxx>]

Hello --

I'm trying to group and sort a list of tasks by category, where one 
<task> may have multiple categories identified by <context> tags. I find 
that I get different results depending on the ordering in the source XML:

<tasks>
   <task>
       <subject>This is to be done at home</subject>
       <context>home</context>
   </task>
   <task>
       <subject>No context so skip this one.</subject>
   </task>
   <task>
       <subject>Make a phone call</subject>
       <context>call</context>
   </task>
   <task>
       <subject>This is to be done at home, and is a phone call</subject>
       <context>home</context>
       <context>call</context>
   </task>
   <task>
       <subject>This is a call to be made from work</subject>
       <context>work</context>
       <context>call</context>
   </task>
</tasks>

Using the following XSLT, I get what I want:

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform";>
   <xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
   <xsl:key name="tasks-by-context" match="task" use="context"/>
   <xsl:template match="/">
       <ul>
           <xsl:for-each select="//task[count(. | 
key('tasks-by-context', context)[1]) = 1][count(context)>0]">
               <xsl:sort select="subject"/>
               <xsl:variable name="c" select="context"/>
               <li>
                   CONTEXT: <xsl:value-of select="$c"/>: (<xsl:value-of 
select="count(key('tasks-by-context',$c))"/> tasks)
                   </li>
               <ul>
                   <xsl:for-each select="key('tasks-by-context', $c)">
                           <li>
                                   Task : <xsl:value-of select="subject"/>
                           </li>
                   </xsl:for-each>
               </ul>
           </xsl:for-each>
       </ul>
   </xsl:template>
</xsl:stylesheet>

As I'm skipping uncategorized tasks (i.e. with no <context> children), I 
get the following output:

- CONTEXT: call: (3 tasks)
   - Task : Make a phone call
   - Task : This is to be done at home, and is a phone call
   - Task : This is a call to be made from work
- CONTEXT: home: (2 tasks)
   - Task : This is to be done at home
   - Task : This is to be done at home, and is a phone call

However, this behavior differs if the original <task> elements are in a 
different order. For example, moving the "Make a phone call" task to the 
end of the <tasks> list reduces the output to the following:

- CONTEXT: home: (2 tasks)
   - Task : This is to be done at home
   - Task : This is to be done at home, and is a phone call

So it looks like my key definition is no longer picking up "call" as a 
valid key item. I'm actually not sure whether I'm using the <xsl:key> 
tag correctly here -- will it match and index a single <task> node 
multiple times if it contains multiple tags?

Thanks in advance for your time.

Ramon