Interesting. I'd been looking around for cycle detection algorithms and knew I might have forgotten a simpler algorithm. I think that would do very nicely for the pretty printer and most likely the marshalling algorithm as well. We'll see if I can find a way to adapt it to work for iso as well, though on the face of it, it doesn't look that straightforward.
As it happens I provided an implementation of iso at http://arclanguage.org/item?id=17365 :) Let me know if my toy language isn't clear. The basic idea is that you need two hare pointers, one for each arg.
