Re: what to do if id() doesn't recognize xml:id

I'm currently fathoming what's possible with keys as they are really
quite handy.  I'm not sure yet whether I fully understand how they
work.  In a short flash of excitement I thought I could do something
like the following:  If I have data like this

<?xml version="1.0"?>
<?xml-stylesheet type='text/xsl' href='xsl/musx2svg.xsl'?>
<children>
  <mother xml:id="mother1" name="Laura" >
    <child father="father1" name="Kim"/>
    <child father="father2" name="Peter" />
  </mother>
  <mother xml:id="mother2" name="Sarah">
    <child name="Ron" father="father2" />
  </mother>
  <father xml:id="father1" name="Jim">
    <child mother="mother1"/>
  </father>
  <father name="George"/>
</children>

where children can either be stored as child elements of their mothers
or fathers, I could maybe directly select the mother of the current
<child> element like "key('mother',.)" regardless of how they are
stored if I was using a key like

<xsl:key name="mother" match="mother" use="child|//child[@mother =
current()/@xml:id]"/>

But then I realized that not the nodes themselves from the node-set
that's returned by "child|//child[@mother = current()/@xml:id]" would
work as keys, but their string values would.  Unfortunately something
like "generate-id(child|//child[@mother = current()/@xml:id])" would
of course destroy the effect as only for for the first element of the
node-set a key would be generated.

So I figured I could add IDs to all <child> elements in an additional
processing step and use a key like

<xsl:key name="mother" match="mother"
use="child/@xml:id|//child[@mother = current()/@xml:id]/@xml:id"/>

But there are potential problems if there

(Of course in this made up example one could as well use
"(parent::mother|key('id',@mother))".  However my intent is not to
solve a special example case.  I'd like to get a clearer idea of where
keys are applicable and where they aren't.)

Thomas W.