[xsl] groupi-by statements?

[Bruce D'Arcus <bdarcus@xxxxxxxxxxxxx>]

Here's another problem:

      <xsl:for-each-group select="current-group()"
	        group-by="substring(mods:originInfo/mods:dateIssued,1,4)">

One of the good things about the data in question is it's recursive.  
It's also a problem in this context.  The year that is critical to this 
processing can be represented like:

mods:originInfo/mods:dateIssued (as above)
mods:relatedItem/mods:originInfo/mods:dateIssued (the date issued for a 
book, where the main record is a chapter)
mods:relatedItem/mods:part/mods:date (the sort of date you may have 
with a journal article; associated with the issue)

If I have more than one of these expressions in my group-by statement, 
I get a "A sequence of more than one item is not allowed as the first 
argument of substring()" error.

So how should I handle this?  Should I define a function -- called, 
say, bib:year -- that I can call like so?

      <xsl:for-each-group select="current-group()"
	        group-by="bib:year()">

Bruce