Hmmm. So how about looking for only unbound single letter symbols? That would cut out all of the predefined functions, and still provide usability (I wasn't going to use it with anything more than x y z and a b c anyway.
If not, I guess there's no point in continuing to pursue this idea, is there ;)
Maybe we could look at only single letter symbols, or symbols that start with _. That would give compatibility with (most) of the current uses; the only times that it wouldn't work would be when it was used in a function that had a single letter parameter. So, maybe I should just give up then ;) Even though I don't really like _1 _2, etc., I guess it's the most viable option.