Gregory's Server@neauoire It's a really subtle case of multi-set rewriting. Because rewrite rules can be formulated over any kind of structure, it's easy to swap other structures in.
FRACTRAN, Petri Nets, some fragments of linear logic, and Nova all share the backing structure of "an unordered bag of things, which can have duplicate items".
It's really neat once you start examining the consequences, because if the things in the bag have some structure, you can pattern-match them.
Then anything is possible.
@wryl It's like I told you before, I'm really bad at understanding anything until I spend way too many hours trying to work it out mechanically. It finally clicked last night what "an unordered bag of things " actually mean. I get why you can't order them now.