[XeTeX] newcommand with optional arguments

[Morten Høgholm]

On Tue, Mar 4, 2008 at 2:35 PM, Bruno Voisin <bvoisin at mac.com> wrote:

> Motivated by your answer I just had a look at the index of the
> TeXbook, directing for \\ to section 2 "List macros'" of Appendix D.
> It seems \\ by default does nothing: as you said it is just a marker
> which is not a character and is used as a delimiter for list items,
> but it can also be redefined by macros to perform specific actions on
> these items.

Knuth uses it as a dummy marker in lists in the examples but \\ is
actually defined in plain TeX and is one of Don's little jokes. He
explains early on in the book that you don't get a backslash from
typing \\ as you should reserve that one for more important things
(paraphrasing here). The definition from plain.tex (l.682) is
  {\catcode`p=12 \catcode`t=12 \gdef\\#1pt{#1}} \let\getf at ctor=\\
The usual error will be a `Runaway argument ...' because p's and t's
with catcode 12 are rarely seen in a document. A jester, I told you.
:-)

-- 
Morten