Sub Articulo 70.0

 I prepare my lecture notes and handouts in latex.  One feature I would
 love to have is to selectively surpress the printing of some of the
 materials.  Something like a ''hide'' macro style would do.  Consider
 the following example:
 
 \usepackage[on]{hide}
 ...
 Q: Why is the sky blue?
 \begin{hide}
 A: Because someone painted it blue.
 \end{hide}
 ...
 
 When I latex it with the on switch, the answer part would be printed;
 but if a off switch is selected instead, the answer part would be
 ignored by latex and hence not printed.  With this setup, on means
 lecture note for me, off means
 handout for students.
 
 So my question: Does there exist such latex style?