(*  CD Management Program Data Dictionary 
*) 

object Data_Base = albums:Album*;

object Album = artist:string and title:string and media:string;
object CD extends Album = song:string;

operation Backup(db:Data_Base, c:CD)->(db':Data_Base)
      precondition: (* The input CD is in the input DataBase *)
        (exists (a in db.albums) (c = a.<CD)) and (c.song = "whatever");
-- WAS: (c in db.albums)                      and (c.song = "whatever");
      postcondition:  ;
end Backup;