You get
$ saxon filter.xml filter.xsl
<?xml version="1.0" encoding="UTF-8"?>
<Documents>
<Document name="sub">
<Article title="1.1" info="sub" filter="food"/>
</Document>
<Document name="main">
<Article title="1.2" info="main" filter="food"/>
<Article title="2.2" info="main" filter="food"/>
</Document>
<Document name="child"/>
</Documents>
because of the different ways you filter.
on document you allow "" but in
<xsl:copy-of select="key('by-info',@info)[(_at_)filter=$filter]"/>
you don't.
I think that _is_ what you want, so in your for-each,
<xsl:for-each select="Document[(_at_)filter='' or
@filter=$filter]/Article[generate-id()=generate-id(key('by-info',@info)[(_at_)filter=''
or @filter=$filter])]">
you want to make sure that you always have at least one eleemnt in the
key that really has the filter value and not just "" so that
<xsl:copy-of select="key('by-info',@info)[(_at_)filter=$filter]"/>
always produces something.
you can do that by making the filters on the key() match,
delete @filter='' or
<xsl:for-each select="Document[(_at_)filter='' or
@filter=$filter]/Article[generate-id()=generate-id(key('by-info',@info)[(_at_)filter=$filter])]">
and you get
$ saxon filter.xml filter.xsl
<?xml version="1.0" encoding="UTF-8"?>
<Documents>
<Document name="sub">
<Article title="1.1" info="sub" filter="food"/>
</Document>
<Document name="main">
<Article title="1.2" info="main" filter="food"/>
<Article title="2.2" info="main" filter="food"/>
</Document>
</Documents>
David
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