I found at least 9 Arc Lisp implementations on the GitHub, some of them seems outdated, but probably this is just because there is small codebase and all obvious bugs are already cleared.
Here is the list, sorted by update time:
* https://github.com/arclanguage/anarki - community managed fork of the Arc3.1 (5 days).
* https://github.com/akkartik/arc - evolving fork of Arc3.1, with keyword arguments and generic functions (2 months).
* https://github.com/arclanguage/arc-nu - attempt to completely rewrite Arc3.1 in racket (3 months)
* https://github.com/awwx/ar - attempt to create a framework for languages like Arc (1 year)
* https://github.com/smihica/glaze - Arc version, written in C++ (2 years).
* https://github.com/conanite/rainbow - version of Arc, written in Java (3 years)
Right now I only tried an Anarky and Arc-nu, trying to write a simple web crawler. And was surprised that Arc-nu not only hasn't a simple http client but even no way to connect to the socket.
The assertion error is a stack overflow. :) At present Arcueid makes use of a fixed-size stack within its virtual machine, and since the compiler still can't properly optimise tail recursion, your code overflows the stack.
Well, I haven't tried to run crawler with large depth setting yet. I discovered, that it have strange problems with threading and almost unable to process in parallel. Then I tried to investigate why. Will make a separate post on it, when will get some valuable information.