vocabulary Monogamy;

/*

* Value Types
*/

Name is written as String; Person ID is written as Auto Counter;

/*

* Entity Types
*/

Person is identified by its ID; Person is called one Name;

Boy is a kind of Person;

Girl is a kind of Person; Girl (as Girlfriend) is going out with at most one Boy (as Boyfriend) [symmetric],

Boyfriend is going out with at most one Girlfriend;

/*

* Constraints:
*/

either Person is a Girl or Person is a Boy but not both;