Add:
* commentary:true (posts with commentary)
* commentary:false (posts without commentary)
* commentary:translated (posts with translated commentary)
* commentary:untranslated (posts with untranslated commentary)
* commentary:"text" (posts where any commentary field matches "text")
Known issues:
* There's no way to escape the true, false, translated, or
untranslated keywords to do a literal text search for commentaries
containing one of these keywords.
* Negated searches may be slow. Using a left outer join instead of a
subquery would be faster in most cases, but negating it is harder.
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