Well, my fourth check did the trick. Thanks, everyone. @KreherFA, @AlexM, @SDDA, and @DocWiley, I understand what led me half-astray. One of the requirements (I won’t say which), if interpreted as surely intended, eliminates the invalid one out of the two solutions I was seeing. It really is a slightly subtle thing, but I should have seen it earlier. Mea culpa! Thanks again, folks, and sorry for the hassle. You were right. I’ll delete my earlier posts in a moment. 
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