Oops ... my post is confused, sorry :)
1. shorthand for "true" - no problem IMO
2. undefined returning "false" ... I don't like that, it's inconsistent with the rest of the templating system and, as Ken says, makes you vulnerable to simple typing errors.
3. dropping "not nil"? Is it really needed? All I should need to say is "if @foo@ true" and if that isn't working correctly then something's broken IMO.
