Global and local definitions of non-functions/variable (was: More re odditie [Was: regular expressions that match nothing])

[Stefan Monnier]

> I would much rather see a clean, robust and expressive extension mechanism
> for rx (`rx-constituents' does not count).  There are various hacks and
> libraries but it probably needs to be integrated.

Agreed.  Something like `rx-defmacro` and `rx-macrolet`.

Reminds me that this is a recurring need (we see it in `pcase`, in
`peg`, here, and arguably in `gv`).  It would be nice if we could design
a general solution that those packages can (re)use.


        Stefan


PS: Arguably `cl-defmethod` could also be extended to a kind of
    `methodlet` for scoped methods, but it might be tricky to do that.
    OTOH, this might provide exactly the generic mechanism we need to
    implement the others.