Using sibling value in streaming mode

Hi,

I've started looking into streaming recently (using Saxon 9.9). I have
a use case like this:

Input:

<array>
    <map>
       <string key="key1">value1</string>
       ...
       <string key="id">123456789</string>
       ...
       <string key="keyN">valueN</string>
    </map>
    ...
</array>

Required output:

<items>
    <item>
       <id>123456789</id>
       <key>key1<key>
       <val>value1</val>
    </item>
    ...
    <item>
       <id>123456789</id>
       <key>id<key>
       <val>123456789</val>
    </item>
    ...
    <item>
       <id>123456789</id>
       <key>keyN<key>
       <val>valueN</val>
    </item>
    ...
</items>

The value of <string key="id"> is used as <id> in <item> elements. The
problem is that <string key="id"> can occur in any position in the
<map>.

I've tried using an accumulator such as

<xsl:accumulator name="map-id" initial-value="()" streamable="yes"
as="xs:string?">
   <xsl:accumulator-rule match="/array/map/string[@key = 'id']/text()"
select="string(.)"/>
</xsl:accumulator>

and then

<item>
    <id><xsl:value-of select="accumulator-before('map-id')"/></id>
    ...
</item>

That worked partially -- only for sibling <string> elements that
followed the <string key="id">. Which is not surprising.

I've also tried accumulator-after('map-id') but got:

  XTSE3430: Template rule is not streamable
  * A call to accumulator-after() is consuming when there are no
preceding consuming instructions

Is it possible to have a streaming solution in this case?

Martynas