title: Why does apple use NSOrderedSet instead of NSArray for ordered mutli value relationship in CoreData?
body: <p>In core data if we have multi value relationship, such as when a business can have several keywords,  in the subclass NSManagedObject generator, Apple will use NSOrderedSet instead of NSArray.</p>
  
  <p>They are both almost equal except that checking ownership is slightly faster in NSOrderedSet.</p>
  
  <p>Any reason why Apple doesn't use the far more familiar and famous NSArray?</p>