Clarification on Wiki Math Macros information

Hi, the Wiki on Math Macros has the following piece piece of information:

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  Changing an existing command

Math-macros translate to \newcommand, i.e. they fail, if the macro name is 
already defined.

To change an existing command (and its rendering in Lyx) you can use the 
following trick: Assume you want to redefine \Re to print “Re” in roman font.

    * define the macro: M-x math-macro Re
    * fill it with \Re in the latex-cmd box and with \mathrm{Re} in the 
lyx-render box
    * put it in a Comment, so latex will not see it
    * do the redefinition in ERT (of course not in a comment): 
\renewcommand{\Re}{\mathrm{Re}} 

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I do not understand how I can "put it in a Comment, so latex will not see it".
I know latex, and I know what a comment in latex is, but what do I do in
LyX to put in a comment.  The user guide says there is a comment paragraph 
environment, which I interpreted to mean that in the pull-down down
where Standard, Caption, Itemize, etc. are I would see a Comment section
but I can't seem to find it.  I am using the revtex4 style.

Thanks