Will Robertson wrote:
> So, to perhaps summarise:
> 
> m M - mandatory braced argument
> d D <x><y> - optional delimited argument; picks up material between <x>
> and <y>. Can be used for {}, (), etc. Can be followed by a default value
> to use.
> o O - shorthand for d[]
> t <x> - lookahead for optional <x>
> s   - shorthand for t*
> l L - everything before the next brace (not often used, but sometimes
> useful)
> 
> No longer:
> 
> c<x><y> - as d() but mandatory; can perform the check manually (only a
> small processing overhead?).
> p - shorthand for d(); drop this to keep things simple.

All fine with me: I will probably look at re-working this into
xparse-alt at the weekend.

> Open issues:
> 
>  - Do we want to support something like 'd{[[}{]]}{default}' to allow
> \foo[[bar]] ? (I'd say no.)

I think that "d" is pretty clearly for delimiting by two *tokens*.
Anything beyond that is not supported (especially if we want to
implement d{} as an optional argument using braces).

>  - Do we want to support something analogous to
>        \def\foo #1\endfoo{...#1...}
>    Consider active quotes, and macros like '\<...>'
>    Maybe this is as important to support as 'l'.
>    Could use the letter 'u{<token>}' for "until".

Perhaps. I'm not keen on active quotes, but I can see the second form
might be useful.  Should not be too hard to do.
-- 
Joseph Wright