I spent most of the Sunday afternoon trying to understand why defining

\newcommand{\era}{\end{comment}}

did not have the same effect as writing the line

\end{comment}

until I found there is a clash due to the comment package… The assuredly simple code

\documentclass{book}
\usepackage{comment}
\begin{document}

\begin{comment}
prompt, you could embark on an on-line visit of the main features of {\tt R} by typing \verb+demo()+ after the prompt (make sure to test \verb+demo(image)+ and \verb+demo(graphics)+ to get an idea of the
\era

Self-explanatory.
\end{document}

produces an error message:

Runaway argument?
! File ended while scanning use of \next.
<inserted text>
                \par
<*> moretest.tex

This is quite an inconvenience as I need to compile my solution manual for “Introducing Monte Carlo Methods with R” with the even-numbered exercises commented out or not depending on the version… (Leaving this package out and using the comment command within the verbatim package does not work either because era does not seem to be recognised as the end of a commented part…)