1. A method of enforcing a constraint on a constrained object in a database system, the method comprising the steps performed in the database system of: receiving a specification in the database system of the constrained object and a specification of an arbitrary constraint set of objects, the objects in the constraint set being possible values of the constrained object; associating the constrained object with the constraint set; and when an operation in the database system adds a value to the constrained object, permitting the operation only if the added value is a member of the constraint set.