James,

 > I've been filling in the gaps in xcontents.dtx, and have a working
 > package that more or less matches the documentation.  The main thing I

sounds great. i guess you are going to share that with us at some point, yes?

 > think from using the new packages it the error catching needs to be
 > greatly improved.

error catching is an important aspect in its own right; i agree. but
for prototype implementations i don't mind much if errors aren't
caught.

after all, the main purpose of prototype impls is that one is prepared
to throw them away and do a clean reimplentation afterwards when the
concepts are stable (eg xparse already has its third impl and is likely
to get another one or two)

 >
 > There is mention in xparse.dtx (I think) of putting help texts in
 > \DeclareDocumentCommand, is that likely to happen?

i would say yes. but one needs to think about how to struture that
"help info" so that external programs (like emacs or whatever) can
make good use of the information. for LaTeX itself the information is
not likely to be usable (i think)

 > Similarly for an
 > informal description of the arguments in \DeclareTemplateType.  Are
 > there any more thoughts on how that could be done?  It would be

not yet. i still change my mind (daily?) about how to present the
information in the documentation. as long as i don't even have a good
idea how to do this on this level ...

but perhaps somebody has some good idea for providing such information.

effectively all i think commands like \DeclareUserCommand or
\DeclareTemplateType etc. should do is to provide an extra argument in
which this information goes but not to use it themselves.

by the way, was this mail sent before it was finished? my copy ends with

 > there any more thoughts on how that could be done?  It would be

which doesn't look like a full sentence

frank