Arc 3.1 takes 80 lines to implement expand=, but Nu takes only 31. And Nu is much clearer and easier to understand as well. In addition, Nu's output is much shorter and is faster:

  ;; Nu
  > (macex1 '(= foo!bar 5))
  (do (sref foo 5 (quote bar)))

  ;; Arc 3.1
  > (macex1 '(= foo!bar 5))
  (do (atwith (g1 foo g3 (quote bar) g4 5) ((fn (g2) (sref g1 g2 g3)) g4)))

* [1]: setforms is included in compat.arc for backwards compatibility with Arc 3.1, but it's not actually used anywhere.