> When using XSLT 1.0 I got various error messages with the 
> above syntax.
> XPath error : Invalid expression
> if (exists($ig)) then $ig/length else 0
>                  ^

Yes, sorry, I'm very out of practice with writing 1.0 code. You have to
expand the XPath if()then ...else to an XSLT xsl:choose. The exists()
function can often be replaced with boolean() or just omitted

<xsl:choose>
  <xsl:when test="$ig">
    <xsl:value-of select="$ig/length"/>
  </xsl:when>
  <xsl:otherwise>0</xsl:otherwise>
</xsl:choose>

or there's often a less verbose way, as you say here using string-length.

Michael Kay
http://www.saxonica.com/