NSAssert( [self.delegate conformsToProtocol: @protocol( UISearchBarDelegate )], @"Some Error." );

Protocol *counter = @protocol(ReferenceCounting);

@protocol ReferenceCounting

- setRefCount:(int)count;

- (int)refCount;

- incrementCount;

- decrementCount;

@end

@interface Formatter : NSObject < Formatting, Prettifying >

@end

if ( [receiver conformsTo:@protocol(ReferenceCounting)] )
   [receiver incrementCount];

@protocol B;

@protocol A
- Foo:(id <B>)anObject;
@end