> It is more or a warning, since this is a common source of bugs. Also, it can
> help to think more about mutability and immutability. I have seen many people
> stumble over this.

OK, but then IMHO it should be formatted as a warning, and maybe a small
discussion on why people should do it should be added (optionnaly using
the 'tip' directive, so that this discussion does not appear on screen
when doing interactive teaching.