I'm a teaching assistant for a programming languages class this quarter, and it just occurred to me while looking at some [EBNF grammars](https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form) that we can get a noncommutative (and asymmetric!) monoidal preorder by considering _ordered_ complexes formed from the combined set of terminals and nonterminals in these grammars. Despite being asymmetric, this still _feels_ resource-y because we're consuming nonterminals to produce (nonterminals and) terminals. This seems to correspond with the notion of a [semi-Thue system](https://en.wikipedia.org/wiki/Semi-Thue_system).

There's also a very enjoyable puzzle of this type from Douglas Hofstadter's _Gödel, Escher, Bach_, called the [MU puzzle](https://en.wikipedia.org/wiki/MU_puzzle). The puzzle asks: given a set of four rules (akin to our "reactions" here), is it possible to get the string MU starting from the string MI? This is just another kind of reachability problem.