Hello, I was trying to model a server that has messages, and want each message in that server to be created by a server member. How can I check this? I specifically am trying to avoid making `relation creator: user` because it implies that a non-member could make a message. I believe the generic question is around subsets? ``` definition user {} definition server { relation member: user } definition message { relation creator: server#member // How can I get this to be a subset of server#member ? relation channel: server } ```