Category usage in Objective-C

@interface SomeClass (private)

@interface NSString (Capitals)

-(NSString *)alternateCaps:(NSString *)aString;

@end

@implementation NSString (Capitals)
-(NSString *)alternateCaps:(NSString *)aString
{
    // Implementation
}

// someClass.m
@interface someClass (extension)
-(void)extend;
@end

@implementation someClass
    // all the methods declared in the .h file and any superclass
    // overrides in this block
@end

@implementation someClass (extension)
-(void)extend {
    // implement private method here;
}

// someClass.m
@interface someClass ()
-(void)extend;
@end

@implementation someClass
    // all the methods declared in the .h file and any superclass
    // overrides in this block
    // Implement private methods in this block as well.
-(void)extend {
    // implement private method here;
}

@end