; definition of falseness for Arc if.
; must include '() since sometimes Arc functions see
; Scheme lists (e.g. . body of a macro).
So '() is ar-false? to interact nicely with Scheme. Notice #f is ar-false?, too.
I don't think there's much to it: pg just seems to prefer to spell the empty list nil instead of (). The only place I can think of where these different spellings are useful is in argument lists. That is, I prefer
(def f () something)
(def f nil something)
Otherwise, I'm used to nil, though I'd probably get used to () just as well.