Suggested by Janet, after inhaling paint fumes. Unlike mother-in-law,
which is an entry in the hallucinating monsters list and would be pluralized
if chosen as a tin description, I think the rank title man-at-arms will only
ever go through plural/singular handling if used as a fruit name. But since
the man/men part was already implemented for pluralization, adding the -at-
part is trivial. Also adds men/man singularization for the general case
where -at- isn't involved.
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