Yesterday's idea has inspired me as much as to create a prototype implementation of map and tuple in the xslt 2.0.

Definitely I wished these were a built-in types, and were considered as atomic values for purposes of comparasions and iteration. This way it were possible to create highly efficient grouping per several fields at once.

This pure implementation (xslt-tuple.zip) is rather scetchy, however it allows to feel what can be done with tuples and maps. I guess a good example may say more than many other words, so have a pleasure:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:f="http://www.nesterovsky-bros.com/xslt/functions"
  exclude-result-prefixes="xs f">

  <xsl:include href="tuple.xslt"/>
  <xsl:include href="map.xslt"/>

  <xsl:template match="/">
    <root>
      <xsl:variable name="tuples" as="item()*" select="
        f:tuple
        (
          for $i in 1 to 10 return
            f:tuple(1 to $i)
        )"/>

      <total-items>
        <xsl:sequence select="count($tuples)"/>
      </total-items>
      <tuples-size>
        <xsl:sequence select="f:tuple-size($tuples)"/>
      </tuples-size>
      <sums-per-tuples>
        <xsl:for-each select="1 to f:tuple-size($tuples)">
          <xsl:variable name="index" as="xs:integer" select="position()"/>

          <sum
            index="{$index}"
            value="{sum(f:tuple-items(f:tuple-item($tuples, $index)))}"/>
        </xsl:for-each>
      </sums-per-tuples>

      <xsl:variable name="cities" as="element()*">
        <city name="Jerusalem" country="Israel"/>
        <city name="London" country="Great Britain"/>
        <city name="Paris" country="France"/>
        <city name="New York" country="USA"/>
        <city name="Moscow" country="Russia"/>
        <city name="Tel Aviv" country="Israel"/>
        <city name="St. Petersburg" country="Russia"/>
      </xsl:variable>

      <xsl:variable name="map" as="item()*" select="
        f:map
        (
          for $city in $cities return
            ($city/string(@country), $city)
        )"/>

      <xsl:for-each select="f:map-keys($map)">
        <xsl:variable name="key" as="xs:string" select="."/>

        <country name="{$key}">
          <xsl:sequence select="f:map-value($map, $key)"/>
        </country>
      </xsl:for-each>
    </root>
  </xsl:template>

</xsl:stylesheet>