How to call objc_msgSendSuper()

AppDelegate.m

//
//  AppDelegate.m
//  CallSuperMethod
//

#import <objc/objc-runtime.h>
#import "AppDelegate.h"

@interface Superclass : NSObject
- (NSString*)a;
@end

@implementation Superclass
- (NSString*)a;
{
    return @"Super";
}
@end

@interface Subclass : Superclass
- (NSString*)a;
@end

@implementation Subclass
- (NSString*)a;
{
    return @"Subclass";
}
@end

@implementation AppDelegate
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    Subclass *subclass = [Subclass new];
    NSLog(@"[subclass a]:%@",[subclass a]);
    struct objc_super superInfo = {
        subclass,
        [subclass superclass]
    };
    NSLog(@"[[subclass super] a]:%@",objc_msgSendSuper(&superInfo, @selector(a)));
}
@end