Chris Hostetter and Yonik's MissingStringLastComparator looks like a neat way to specify where to put null values when you want them to appear at the end of reverse sorts rather than at the beginning, but I spotted the note...
// Note: basing lastStringValue on the StringIndex won't work // with a multisearcher. Is that a show-stopper for MultiSearchers, or does it just mean that it is a bit less efficient?
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