On Fri, Dec 01, 2006 at 02:48:39PM +0100, Jean-Marc Lasgouttes wrote:
> >>>>> "Enrico" == Enrico Forestieri <[EMAIL PROTECTED]> writes:
>
> Enrico> Now, when \csname is used to define a command for the first
> Enrico> time, this command is made equivalent to \relax until it is
> Enrico> redefined.
>
> Ahh, I did not know that \csname...\endcsname could define the macro
> if it did not exist...
This is what the TeXbook says:
When TeX expands \csname it reads to the matching \endcsname, expanding
tokens as it goes; only character tokens should remain after this
expansion has taken place. Then the `expansion' of the entire
\csname...\endcsname text will be a single control sequence token,
defined to be like \relax if its meaning is currently undefined.
> Enrico> For example, one could define such a thing:
> Enrico> \def\ifundefined#1{\expandafter\ifx\csname#1\endcsname\relax}
> Enrico> to do something like \ifundefined{foo}...\else...\fi (This is
> Enrico> exercise 7.7 in the TeXbook).
>
> But in this case \ifundefined is non-const (in the c++ sense), isn't it?
Yes, after that you will find that you have a \foo command equivalent
to \relax.
> Enrico> Moral is that you have to define \endXXX only if you have to
> Enrico> actually do something when the environment XXX ends.
>
> OK.
\begin{em}emphasized\end{em} and not emphasized.
It works...
--
Enrico
