> Macro: (thunk body). It's simple enough. Just like the square bracket syntax covers one-argument functions, this can cover zero-argument functions.
A side effect of making all function arguments optional  is that the square bracket syntax can then cover zero-argument functions too, so we get this additional (shorter) way to express your thunk example:
Actually, I don't use 'thunk when it doesn't save me parens. Both "thunk" and "fn ()" take up the same number of characters (albeit not the same number of tokens), and I figure Arc programmers will have to look up what 'thunk means more often than they look up 'fn. :-p
Incidentally, it actually does save parens in that case: