wrapping my head around subrelations. i see this in the docs:

definition group {
    /**
     * member can include both users and *the set of members* of other specific groups.
     */
    relation member: user | group#member
}

does that mean that a group is also a valid object of a relation, but that that group's presence on the relation will be interpreted to mean the set of members? or is it that the subrelation is also what you'd declare as the object of the relation?