A question about a macro definition errors

Question

Given:

\documentclass{minimal}
\newcommand{\emrule}[1]{\rule[4pt]{#1em}{0.2pt}}
\newcommand{\endash}{\emrule{.5}}
\begin{document}
2000\endash2013
\end{document}

I get:

! LaTeX Error: Command \endash 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.3 \newcommand{\endash}{\emrule{.5}}

? 

So my first question is what manual---see p.192 of the manual.? Through experimentation I believe that what is going on here is a name clash. The processor sees \end and hits the brake with an attendent error message. For instance \EMdash and \enrule both work without error. Since I lack sufficient macro-foo, could someone with greater knowledge than I please clarify? BTW if I comment out the definition of \endash it fails with a message about \endash not defined:

! Undefined control sequence.
l.5 2000\endash
               2013
? 

Which leads me back to my contention that this is a name clash with just \end.

Answer 1

\newcommand explicitly refuses to define macros whose name begins with end. This may have been a disputable design decision, but the LaTeX kernel is kind of frozen and changing this behavior after so many years would open big problems. With \NewDocumentCommand from the xparse package you're freed from this restriction:

\usepackage{xparse}
\NewDocumentCommand{\endash}{}{\emrule{.5}}

Another way can be to test if the command already exists in a different way:

\unless\ifdefined\endash
  \def\endash{\emrule{.5}}
\fi

relying on lower level TeX constructions.

Are there any problems with this? Yes, there are. If you later on say

\newenvironment{ash}
 {something maybe related to cricket at the start}
 {something else at the end}

you'd find yourself in big trouble, because this declaration would silently redefine your \endash command: indeed the ash environment uses \ash and \endash at start and finish.