@Diag @Use clause

[Jeff Kingston]

> Or you can put the @Use clause for @DiagramPrint before the @Use
> clause for @DocumentLayout, since the two packages are completely
> independent.  Hmm, this seems like a good idea to do by default in d*
> setup files.  Jeff, what do you think?

Interesting idea.  We already have some ideas for making setup files
more flexible, by breaking up @DocumentLayout into several smaller
pieces, each of which would then be able to supply symbols for use
by the next.  So this idea of @Diag needs to be incorporated into
that line of work (which I haven't started yet, but do intend to do).

I've been pondering ways to reduce the combinatorial explosion of
setup files.  It occurred to me that @Use could be redefined as a
no-operation if the symbol inside it was undefined.  In that way
we could have just one setup file for each type of document, but
still not pay the cost of reading lots of unwanted definitions
files, just quickly skipping over their @Use clauses.  (Another
penalty incurred when reading, say, the @Diag definitions is that
you automatically get a large header on your PostScript output
containing @Diag's PostScript "runtime system".)

Jeff