We have found that Saxon HE 9.7.0-18 has finally exposed partial support to map and array item types. So, now you can encapsulate your data in sequence rather than having a single sequence and treating odd and even elements specially.

Basic example is:

<xsl:stylesheet version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:t="t"
  xmlns:map="http://www.w3.org/2005/xpath-functions/map"
  exclude-result-prefixes="xs t map">

  <xsl:template match="/">
    <xsl:variable name="map" as="map(xs:string, xs:string)" select="
      map 
      {
        'Su': 'Sunday',
        'Mo': 'Monday',
        'Tu': 'Tuesday',
        'We': 'Wednesday',
        'Th': 'Thursday',
        'Fr': 'Friday',
        'Sa': 'Saturday'
      }"/>
      
     <xsl:message select="map:keys($map)"/>
  </xsl:template>  

</xsl:stylesheet>

A list of map functions can be found here http://www.w3.org/2005/xpath-functions/map/, though not all are available, as Saxon HE still does not allow inline functions.

P.S. From the development perspective it's a great harm that Saxon HE is so limited. Basically limited to xslt 2.0 + some selected parts of 3.0.