How to create an empty macro?

[Jean-Marc Lasgouttes]

Hi there,

I tried to replace the example in section 6.9 Branches of the user's 
guide by proper math macros, because the example as it is breaks previews.

So I would like to define
\newcommand[1]{\question}{#1}
\newcommand[1]{\answer}{}

However, I cannot construct a math macro that does not use its 
parameter. The parameter is automatically removed. Is there a way around 
this?

I guess my best solution is to find a better example in this case.

JMarc