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

[Drew Adams]

Without wanting to butt in here, and knowing
nothing about what rx offers...

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?

If so, are the things in that subset identified
as such in the doc?

Just wondering.  Wondering, in part, whether
use of rx might indirectly help someone learn
about Emacs regexp syntax and behavior.

Not saying that's important.  Just thought it
might be of some use, or at least interesting.