%%% Null control sequence

When you do:

\beginverbatim
\endlinechar=-1
\let\x\
\endverbatim

the macro \verb'\x' is undefined\dots

Actually \verb'\x' becomes equal to the {\tt null control sequence} that
you would get from 

\beginverbatim
\expandafter\def\csname\endcsname{}
\endverbatim

but that is usually undefined.

And you can even use this effect to assign to the {\tt null control
sequence} without needing \verb'\expandafter':

\beginverbatim
\endlinechar=-1
\gdef\
	{\message{NULL CS}}

\csname\endcsname
\endverbatim

\endinput
