Well, one thing I've occasionally considered is having string?b become [isa b 'string]. :-p That would only do so much though, since you can just define (def a- (type) [isa _ type]) and say a-!string.b.
A while ago I was thinking of using a?b to mean (b a), with the question mark acting as sort of a backwards-pointing period. Now, for Penknife, I'm using a'b for that purpose, since it's sorta an upside-down period and it doesn't require the shift key (or even reaching off of the home row). However, a'b is parsed as two expressions in Arc, so using a`b or a?b for this purpose would probably be easier.
