Yeah, what Daniel said. My code would compile but not work in 9.3/6M62. To my knowledge, relations as properties don’t work at all before 10.1, and in 10.1’s two years now of existence, I don’t think I’ve seen anyone else use them.
That said, Graham’s 2020 talk at Narrascope put forth:
Inform’s “Everything is an X”: relations and rules
[…]
For Inform, there are two big ideas: relations and rules.
I think it’s the case that something like my solution is actually “meant to” be the Inform-ish approach to a problem like this (or, at very least, that there’s a defensible argument for such).
Unfortunately, the docs are so opaque and omit so much detail you need to actually have a shot of knowing what’s possible and how to express that in code, that relations’ use tends toward the simplistic.