bug#37659: rx additions: anychar, unmatchable, unordered-or

[Drew Adams]

> > Is there an identifiable subset of rx features
> > (operators, functions, thingies, or whatever
> > its composable pieces are called) that map
> > (even if not one-to-one) to regexp syntax
> > components?
> 
> Almost all of rx maps one-to-one to the string regexp syntax. The rx
> docs mention the corresponding string regexps for most forms.
> 
> In addition, rx is self-explaining in the sense that a user curious
> about what a particular expression means needs only to evaluate (rx
> SOMETHING) to get the translation. (There are external packages for
> going in the other direction.)

Yes, I knew the last part - rx returns a regexp
(which you can examine).

My suggestion was really about documenting the
correspondence between individual rx components
and regexp components.  If that's already done,
great.  Thx.