https://twitter.com/kain3939/status/425562408594051072　より 1+1を3にしてみた。 ただのMethodSwizzling。あまりエレガントではない。

gistfile1.m

#import <Foundation/Foundation.h>
#import <objc/runtime.h>
#import <objc/objc.h>
#import <objc/message.h>

@implementation NSDecimalNumber (hack)

- (NSDecimalNumber *)decimalNumberByAdding_:(NSDecimalNumber *)decimalNumber
{
    return [[self decimalNumberByAdding_:decimalNumber] decimalNumberByAdding_:(self.integerValue == decimalNumber.integerValue == 1) ? decimalNumber : [NSDecimalNumber zero]];
}

@end

void tarunon()
{
    Class klass = [NSDecimalNumber class];
    method_exchangeImplementations(class_getInstanceMethod(klass, @selector(decimalNumberByAdding:)), class_getInstanceMethod(klass,  @selector(decimalNumberByAdding_:)));
}

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        NSLog(@"1 + 1 = %@", [[NSDecimalNumber one] decimalNumberByAdding:[NSDecimalNumber one]]);
        tarunon();
        NSLog(@"1 + 1 = %@", [[NSDecimalNumber one] decimalNumberByAdding:[NSDecimalNumber one]]);
    }
    return 0;
}