- (instancetype)init NS_UNAVAILABLE;
+ (instancetype)new NS_UNAVAILABLE;
- (instancetype)init NS_DESIGNATED_INITIALIZER;

#import 

@interface Calculator : NSObject

@property (nonatomic) double result;

- (void)pushOperand:(double)operand;
- (void)add;

- (instancetype)init NS_UNAVAILABLE;
+ (instancetype)new NS_UNAVAILABLE;

@end