[xsl] Best practice for group-adjacent with quasi-null grouping key?

["David Sewell dsewell@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

(Following assumes XSLT 2.0)

I don't use the @group-adjacent form of <xsl:for-each-group> often enough to be 
a master of it. I just ran into the problem that an empty sequence is not 
allowed as the result of evaluating the group-adjacent expression. Given data 
like

<contributor role="author">Joe Jones</contributor>
<contributor role="author">Mary Mitchell</contributor>
<contributor role="editor">Betty Berkeley</contributor>
<contributor role="editor">Peter Parker</contributor>

my first instinct was to use
  <xsl:for-each-group select="contributor" group-adjacent="@role">

but I got a run-time error because the @role is not always present in my 
data. So I revised to

  <xsl:for-each-group select="contributor" group-adjacent="if (@role)
    then string(@role) else 'false'">

which give the desired results. But is there a better general strategy, 
maybe using boolean false() as the grouping value, and then being sure 
to cast current-grouping-key() as a string whenever a string comparison 
is needed?

(I see that XSLT 3.0 adds the @composite option to xsl:for-each-group in which 
case the group-adjacent expression is allowed to evaluate as empty sequence, but 
I'm stuck with a version 2.0 processor in my use case.)

David

--
David Sewell, Editorial and Technical Manager
ROTUNDA, The University of Virginia Press
PO Box 400318, Charlottesville, VA 22904-4314 USA
Email: dsewell@xxxxxxxxxxxx   Tel: +1 434 924 9973
Web: http://rotunda.upress.virginia.edu/