depthcharge was: Ignoring pdftex primitives

[Robert Howlett]

Chris Rowley wrote:
> 
> I wrote --
> > Taco wrote --
> > > >
> > > > "widthcharge.png" needs only 1 \noexpand, "heightcharge.png" needs 2
> > > > (but more \noexpands don't do any harm). "./heightcharge.png" needs
> > > > none, of course, but that's not portable across filesystems.
> >
> > Aagh!  I wish you had not told me that:-) ... now I need to completly
> > reconstruct my personal model of how that bit of TeX works
> 
> I'm OK again now with this one ... when it does find a
> <rule dimension>, it starts scanning for width again,
> etc, etc, ad infinitum ... what a lot of scanning!
> This is all as per the syntax on p282.
> 

But when vrule or hrule is followed by noexpand, as with
\vrule \noexpand\noexpand ...,
TeX effectively discards the first noexpand after looking unsuccessfully
for "width", so that it has fewer noexpands in front of it when it
goes on to look for "height". I don't see anything on p282 or
elsewhere in the TeXBook that indicates a lack of symmetry
in the syntax relating to the keywords "width", "height" and "depth",
and so I still contend that TeX's different behaviour with
the two constructions
xxx \vrule \noexpand\empty width xxx
and
xxx \vrule \noexpand\empty height xxx
is a bug.

Bob Howlett