Not quite what I want, need a little more logic.  I've ALMOST got it
now.  I've figured out that I can just use a normal untyped parameter,
pass it as a set, such as:

fn:myconcat(('abc', '123', 'def'))

In the function figured out to use for-each on the parameter, but now
trying to figure out how to tell which element # I'm on inside the
for-each, especially to detect the last item.  Also need to discard the
'current' item if it's empty, which I can't quite get.

T:\ftemp>type chris.xsl
<?xml version="1.0" encoding="US-ASCII"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:xsd="http://www.w3.org/2001/XMLSchema"
                exclude-result-prefixes="xsd"
                xmlns:c="urn:x-Chris"
                version="2.0">

<xsl:template match="/">
  <xsl:value-of select="c:mysequence(('abc', '123', 'def'))"/>
  <xsl:text>
</xsl:text>
  <xsl:value-of select="c:myconcat(('abc', '123', 'def'))"/>
</xsl:template>

<xsl:function name="c:mysequence">
  <xsl:param name="args"/>

  <xsl:for-each select="$args">
    <xsl:choose>
      <xsl:when test=". castable as xsd:decimal">
        <xsl:sequence select="xsd:decimal(.) * 2"/>
      </xsl:when>
      <xsl:when test="position()=last()">
        <xsl:sequence select="concat(.,.)"/>
      </xsl:when>
      <xsl:otherwise>
        <xsl:sequence select="."/>
      </xsl:otherwise>
    </xsl:choose>
  </xsl:for-each>
</xsl:function>

<xsl:function name="c:myconcat">
  <xsl:param name="args"/>

  <xsl:for-each select="$args">
    <xsl:choose>
      <xsl:when test=". castable as xsd:decimal">
        <xsl:value-of select="xsd:decimal(.) * 2"/>
      </xsl:when>
      <xsl:when test="position()=last()">
        <xsl:value-of select="concat(.,.)"/>
      </xsl:when>
      <xsl:otherwise>
        <xsl:value-of select="."/>
      </xsl:otherwise>
    </xsl:choose>
  </xsl:for-each>
</xsl:function>

</xsl:stylesheet>
T:\ftemp>xslt2 chris.xsl chris.xsl con
abc 246 defdef
abc246defdef
T:\ftemp>

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