[texhax] Unexisting commands.

[Philipp Stephani]

Am 14.04.2010 um 13:33 schrieb Paul Isambert:

> Shouldn't we try to all use the same construction to test for the existence of commands?

You should normally treat \relax as undefined, but nevertheless take care not to define control sequences with \csname. Everyone should use packages like etoolbox for these tests because they are not trivial.

> If we took e-TeX for granted, then \ifcsname would solve the problem.

Even without e-TeX, you can check for the existence of control sequences without visible side effects:

\begingroup\expandafter\expandafter\expandafter\endgroup
\expandafter\ifx\csname ...\endcsname\relax ...