[m-rev.] for post-commit review: centralize decisions about mutables' aux preds

[Zoltan Somogyi]

For post-commit review by anyone.

I know the new predicates in set.m need both copying to the other
set modules and an entry in NEWS, but I want to do that in a day or two,
after I propose and get your feedback on a standard order of predicates
in the various set modules.

Zoltan.
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