When trying to create a category for NSInteger, the compiler complains that it "Cannot find interface declaration for 'NSInteger'". Is it not possible to create an NSInteger category?

A simple test will show the compiler error:

```
#import <Foundation/Foundation.h>

@interface NSInteger (NSInteger_Extensions)

-(NSInteger)simpleTest;

@end

#import "NSInteger_Extensions.h"

@implementation NSInteger (NSInteger_Extensions)

-(NSInteger)simpleTest {
    return self + 5;
}

@end

```

Should this be possible?

Thanks!