Tip #3 works great. It is especially easy now for me to create two versions of my tex code that show slightly different content, with only a one-line modification, without having to create two separate files.

http://www.troubleshooters.com/linux/lyx/ownlists.htm

This shows how to do ‘question and answer’ sheets for a multiple choice quiz, again using environments.

You can use if-statements or redefine commands at the start of the document to get different versions of the same document. Eg. embed all answers in a \answer environment and define answer as an empty command or as a command that actually prints the answer…

I’m mostly writing to answer to willygarcia, even though that is a year-old comment now… but in case anyone else is looking, I found the answer to what he wants to do, on another blog:

http://texblog.wordpress.com/2007/12/31/commenting-in-latex/

In case that blog goes away, the commenter was Jonathan Dursi of http://www.cita.utoronto.ca/~ljdursi/ and he said the following:

(AND I QUOTE:)
I’ve grown quite fond of the comment environment, as it allows you to turn on and turn off the commented text. In particular, I now use it for assignments to generate the version with and without solution sets. I put the answers in section blocks like this

\begin{answer}
\end{answer}

and for the version without the solutions, I just include the line

\excludecomment{answer}

To be fancier, and show the answers set off in shaded boxes, I also use the color and framed packages:

\usepackage{color}
\usepackage{framed}
\usepackage{comment}
\definecolor{shadecolor}{gray}{0.875}

as comment allows you to, when showing the commented block, set it in any environment:

\specialcomment{answer}{\begin{shaded}}{\end{shaded}}
(END OF QUOTE.)

Hope that helps, and even more so, I hope the formatting works when I paste his codes here! :)

That is to say I want them in the same .tex project. But with
with some sort of redefinition (most likely redefining an environment) I
would like to show (or not) the answers.

What I have so far is SEARCH AND REPLACE \comment -> \answer
which is not too great.