[texhax] Macro problem after updating xspace

[Uwe Lueck]

"KKThird at Yahoo.Com" <kkthird at yahoo.com> schrieb am 21.09.2006 12:45:07:
 [look up yourself, it seems to have been a HTML mail which I can't insert 
here at present]

The query is an example for quite a number of items in 

    http://catb.org/~esr/faqs/smart-questions.html

I find it difficult to guess what  you have updated and what not, 
what you expect your version of \upsilon to do, 
what works and what doesn't work (the two example files 
are identical), 
and how the problem is related to \bm (where do you want 
to apply it?). 

However in trying myself I found a problem which -- I guess --  
has puzzled you. \xspace involves \protect and \futurelet\@let at token, 
and \bm seems to disable the \protect and tries to expand \@let at token. 
More generally, any attempt of using \protect to execute an assignment 
in typesetting mode only fails within $\bm{...}$. I am preparing a 
latex bug report; you find an example file below. 

This has nothing to do with txfonts, and the same happens 
with earlier versions of \xspace, so updating xspace can't 
be responsible. If any update caused the problem, it may 
have been updating the bm package. I am unable at the 
moment to investigate this. 

For using \xspace together with \bm, you should be aware of 
the following: 
\bm needs math mode, so when \xspace is involved in 
\bm{...}, it has no effect anyway (unless within an \hbox or so).  
Perhaps you should use some \yspace in place of \xspace 
where 

    \newcommand{\yspace}{\relax \ifmmode \else \xspace \fi} 

Hope this helps, 

Uwe. 


%% bmfragil.tex
\documentclass{article}
\usepackage{bm}
\makeatletter
\DeclareRobustCommand{\LettingAt}{\let\@let at token=x}
\newcommand{\FragileLettingAt}{\let\@let at token=x}
\makeatother 
\DeclareRobustCommand{\LettingNoAt}{\let\mylettoken=x}
\newcommand{\ProtectingLettingNoAt}{\protect\FragileLettingNoAt}
\newcommand{\ProtectingLettingAt}{\protect\FragileLettingAt}
\newcommand{\FragileLettingNoAt}{\let\mylettoken=x}
\begin{document}
% $\bm{\LettingNoAt}$
% $\bm{\LettingAt}$
% $\bm{\ProtectingLettingNoAt}$
% $\bm{\ProtectingLettingAt}$
% $\bm{\protect\FragileLettingNoAt}$
$\bm{\protect\FragileLettingAt}$
\end{document}