var x:UserRecord*;

operation AddUser
    inputs: udb:UserDB, ur:UserRecord;
    outputs: udb':UserDB;

    postcondition:
        (*
         * The given user record is in the output UserDB.
         *)
        (ur in udb')

            and

        (*
         * All the other records in the output db are those from the input db,
         * and only those.
         *)
        forall (ur':UserRecord | ur' != ur)
	    ((set x = x + ur') = (x + ur')) and
            if (ur' in udb)
            then (ur' in udb')
            else not (ur' in udb');

end AddUser;

> set x = [];
> x;