Holy crap, how have I never heard of noparen.arc? I'm gonna see how PG's design choices were different from mine in http://akkartik.name/post/wart. Did I do something he didn't consider, or did he dismiss my approach for aesthetic reasons?
In the past I've tried to think about ways to handle it automatically, but I usually get hung up on macros and their pesky propensity for undoing assumptions. E.g., for my Vim ftplugin (https://bitbucket.org/fallintothis/arc-vim), I used the heuristic "is the macro's rest parameter named body?" to figure out the indentation of so-called "bodops"...except that I had to manually maintain white/blacklists of troublesome macros with code similar to http://arclanguage.org/item?id=11288
