[m-dev.] for review: new predicates in map.m

[Andrew Bromage]

G'day all.

Tyson Dowd wrote:

> Well, maybe.  That's one way of fixing the bugs.

I prefer to think of it as codifying the semantics.

> But it's inconsistent with our normal scheme of using the word
> "sorted" to indicate sorted, which "" indicates nothing.

Except, as pointed out earlier, in the case of solutions/2, where ""
indicates sorted and "unsorted" indicates nothing.

Cheers,
Andrew Bromage