The Area being Modelled is :-
A Library of Reference Documents for Lawyers.

The User Requirements have been defined as follows :-
A Database is needed for a law firm to archive their references.

References are one of 3 types:- articles, books and collective works.

There are common attributes to the 3 types, such the Call number, title, date and place of publication.

However, there also attributes that are different. eg : books have an author, physical description, publisher.
Articles have an author, source(where they appear), and page of start and end.
References have subjects or topics for searching.

B. The Things of Interest, ('THINGS'), include :-
B.1 Articles
B.2 Books
B.3 Collective Works
B.4 References
B.5 Topics

C. These THINGS are Related as follows :-
C.1 A REFERENCE can be either an ARTICLE, BOOK, or COLLECTIVE WORK.