Great article! I have been using Lisp languages for much of my work since the early 1980s, and this article is inspiring because instead of just building a "language" up towards the application domain by adding functions and a few macros, Racket supports more general creation of new languages.
I just read the article quickly because today is a busy day, but I'm going to play with this material more this weekend.
Has it moved to Racket, then? As I recall (but I could easily be misremembering) Arc required some old version of PLT Scheme that still came with old fashioned mutable pairs. (And I think they switched to immutable pairs before they switched to the name Racket.)
Last I heard it was still on an old version. Arc really should update. For example, Racket has just changed it's IO system to support epoll/kqueue out of the box. I expect Hacker News would greatly benefit from this.