User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-GB; rv:1.9.2.15)
            Gecko/20110303 Thunderbird/3.1.9
MIME-Version: 1.0
References: <4DA5C4E2.8090005@morningstar2.co.uk>           
            <58AFBC3A-4209-4BC0-BB3A-5B14D6B5EFD8@gmail.com>           
            <4DA727A9.2050903@morningstar2.co.uk>           
            <BANLkTi=gG=oWHUaLyExNwQ6WzghSV6JXyQ@mail.gmail.com>           
            <19880.45400.677093.956908@morse.mittelbach-online.de>           
            <BANLkTimfOA8TkQdB0uxtcFTbm3EqzHEAFw@mail.gmail.com>           
            <BANLkTi=wyCyZvrjZtvc-gqBSugJuFD0pMQ@mail.gmail.com>           
            <4DADDE63.2010304@morningstar2.co.uk>
            <BANLkTikz4DzyccxSwOf8ds0YMktTnAsNog@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Message-ID:  <4DADFA0F.6080100@morningstar2.co.uk>
Date:         Tue, 19 Apr 2011 22:09:35 +0100
Reply-To: Mailing list for the LaTeX3 project
              <LATEX-L@listserv.uni-heidelberg.de>
Sender: Mailing list for the LaTeX3 project <LATEX-L@listserv.uni-heidelberg.de>
From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: The nature of popping from an empty sequence
To: LATEX-L@listserv.uni-heidelberg.de
In-Reply-To:  <BANLkTikz4DzyccxSwOf8ds0YMktTnAsNog@mail.gmail.com>
Precedence: list
Status: R

On 19/04/2011 21:20, Bruno Le Floch wrote:
> Of course, the <value> must be a token list, but the <key> could be
> more restricted?

I thought we were talking about sequences! (I suggest we finish that
discussion before moving on to other things.)

> I'm guessing that the use of a prop may be
> 
> \q_prop name \q_prop {S\o m\c{e}$t_{hi}n^g$}
> \q_prop country \q_prop {Br\'azil}
> \q_prop I.D. number \q_prop {2CUOHE@#@}
> \q_prop ef{}ficiency \q_prop {12}
> (etc.)
> 
> Here <values> may be anything, but <keys> are rather well behaved. The
> key (sic) property of <keys> in my understanding is whether they
> differ or not, and detokenizing will not cause too many collisions?

Within xtemplate, you'll find some places where we use the fact that the
keys do have catcodes to pull off some 'special effects'. Now, we also
remove spaces there, so spaces can also be used for said 'special
effects'. But such a change would require some alterations.
-- 
Joseph Wright