Command \cs already defined in memoir class but not in amsart?

Question

Consider the following code:

\documentclass{memoir}
\newcommand{\cs}{\Sigma}
\title{Rings and Modules}
\author{}
\begin{document}
\maketitle
\end{document}

This fails to compile and I get the following error.

 ! LaTeX Error: Command \cs already defined.
           Or name \end... illegal, see p.192 of the manual.

 See the LaTeX manual or LaTeX Companion for explanation.
 Type  H <return>  for immediate help.
 ...                                              

 l.2 \newcommand{\cs}{\Sigma}

P.S. The code is perfectly fine with amsart class. Also, please tell me what is "the manual" referred to in the error message.

Answer 1

The file memoir.cls has this line

\DeclareRobustCommand{\cs}[1]{\texttt{\char`\\#1}}

which is meant to print control sequence names: \cs{cs} would print \cs. It doesn't seem a crucial command so you can use a known trick to redefine it without worries about it being defined or not:

\providecommand{\cs}{}
\renewcommand{\cs}{\Sigma}

This is safe both with the standard LaTeX classes, AMS classes and memoir. If you use another class, it's best to comment out the first line and see what happens. If the command is defined, try and look at its definition.

Answer 2

The memoir class indeed includes the following definition of \cs:

\DeclareRobustCommand{\cs}[1]{\texttt{\char`\\#1}}

It is used to format in-text-examples of control sequences.

\documentclass{memoir}
%\newcommand{\cs}{\Sigma}
\title{Rings and Modules}
\author{}
\begin{document}
\maketitle
\cs{mycommand}
\end{document}