filtering maps by key characteristics?

Hello --

I have a content set traversal that creates map entries when it finds id
attributes in the source tree; one map entry maps a generated GUID to
the node with the id attribute, and another map entry maps the value of
the id attribute to the same GUID.

The result is stored as a sequence of singleton maps:

<xsl:variable as="map(*)+" name="traversalMap">
  <xsl:apply-templates mode="traverse" select="$contentSet" />
</xsl:variable>

This sequence of ~map entries then get filtered into two merged maps, one with the GUID-to-element mapping, and one with the id-value-to-GUID mapping:

<xsl:variable as="map(xs:string,element())" name="GUID2nodeMap">
  <xsl:map>
    <xsl:sequence select="$traversalMap ! .[starts-with(map:keys(.), 'GUID-')]" />
  </xsl:map>
</xsl:variable>
<xsl:variable as="map(xs:string,xs:string)" name="DITA2GUIDmap">
  <xsl:map>
    <xsl:sequence select="$traversalMap ! .[not(starts-with(map:keys(.), 'GUID-'))]" />
  </xsl:map>
</xsl:variable>

(I thought it would be easier to split a sequence of ~map entries into
two maps than it would be to traverse the tree twice.)

And it works, but it wouldn't work if I was starting with a merged map,
rather than a sequence of ~map entries/singleton maps.

Is there a way to do this with a merged map, that is, filter a map with
multiple entries by testing characteristics of the keys without having
to reconsitute each entry?

$traverseMap[starts-with(map:key(.),'GUID-')]

doesn't work; the context is the WHOLE map, not a map entry.  And there
doesn't seem to be a way to treat a map as a sequence of map entries.

Is there one?  Am I missing something?

Thanks!

-- 
Graydon Saunders  | graydonish@xxxxxxxxx
^fs oferiode, pisses swa mfg.
-- Deor  ("That passed, so may this.")