No subject

"Another use is to group matches in a particular category (e.g. you
might collapse a mailing list search on the Subject: so that there's
only one result per discussion thread). In this case you can use
get_collapse_count() to give the user some idea how many other results
there are. And if you index the Subject: as a boolean term as well as
putting it in a value, you can offer a link to a non-collapsed search
restricted to that thread using a boolean filter."

http://xapian.org/docs/apidoc/html/classXapian_1_1Enquire.html#117ee547f5908e952e2e72d5a986d3bb

This sounds very like the case you're talking about.


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