Nullability Annotations

_Nullable -can have a nil value
-Nonnull -not expected to be nil

@interface Book : NSObject

@property (nonatomic, copy) NSString *title;
@property (nonatomic) Person *author;
@property (nonatomic) Person *editor;
@property (nonatomic) int yearOfPublication;

-(instancetype)initWithTitle:(NSString*)title
	author:(Person*)author
	year:(int)year;

@end