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
}