[erlang-questions] Side-effects in "if" guards

[David Mercer]

> The main snag is that the rules for both syntax and semantics of
> guards is subtly different from those of normal expressions, so
> it's hard to extend what you can write in an if-guard in a way that
> does not change the behaviour anywhere in all the existing code.

What are the subtle differences?  I suspect most new users do not completely
grok that, since this subject comes up repeatedly.

DBM