Each Relationship Is in Two Collections

Each relationship belongs to two collections. For example, the relationship Herman Melville wrote Billy Budd is a member of two different collections:

• The set of books written by Herman Melville; or, expressed in terms of a relationship collection, the set of authorships for which Herman Melville is the writer.
  
  
• The set of authors of Billy Budd; or, expressed in terms of a relationship collection, the set of authorships for which Billy Budd is the written thing.