@namespace(uri="library", prefix="")
package library;

class Library {
   val Writer[*] writers;
   val Book[*] books;
}

class Writer {
   attr String name;
   ref Book[*]#author books;
}

class Book {
   attr String title;
   attr int pages = 100;
   attr BookCategory category;
   ref Writer[1]#books author;
}

enum BookCategory {
   Mystery;
   ScienceFiction;
   Biography;
}