//Objective-C

@protocol Entity<Validatable>
 - (BOOL)isValid;
@end

@interface EntityList<__covariant ObjectType: id<Entity>> {
  Entity* owner;
  NSMutableArray<ObjectType>* list;
}

- (instancetype)initWithOwner:(id<Entity>)anOwner;
- (ObjectType)objectAtIndex:(NSUInteger)index;
- (ObjectType)objectAtIndexedSubscript:(NSUInteger)idx;
- (ObjectType)objectForKeyedSubscript:(id <NSCopying>)key;
- (BOOL)containsObject:(ObjectType)anObject;
- (void)addObject:(ObjectType)anObject;
- (NSUInteger)count;

@end

@interface Item : Entity {
}
@end

@interface ItemList: EntityList {

}
@end


//Swift

let list = ItemList(owner: Item())
let items = list?.count() //'EntityProtocol' cannot be used as a type conforming to protocol 'EntityProtocol' because 'EntityProtocol' has static requirements