I have established a key:
 B <xsl:key name="placement-index" match="Stamp"
use="concat(CatNumbers/@pofis-prefix, CatNumbers/@pofis-number,
CatNumbers/@pofis-suffix)"/>

It uses an XML file that contains the element Stamp. It worked until
today. What changed is that the CatNumbers fall into two Periods
(seedataB  below). Until today, the Catnumbers referenced always fell in
Period domain='c' in my data. They now also fall into Period
domain='cr'.B  I need to restrict the output of the template to only keys
built from the @domain='c' part of the data. Have not a clue how to do
this.

key('placement-index', concat(@pofis-prefix, @pofis-number,
@pofis-suffix), doc('../work/data-with-index.xml')//Period[@domain = 'c'])