they make it impossible to test which a collection is.
// NSMutableDictionary [dict respondsToSelector:@selector(setObject:forKey:] // NSMutableArray [arr respondsToSelector:@selector(addObject:)] // NSMutableString [str respondsToSelector:@selector(replaceCharactersInRange:withString:)] // NSMutableSet [set respondsToSelector:@selector(addObject:)]
Example SO question: http://stackoverflow.com/questions/1788690/objective-c-how-t...
Another example discussion: http://www.cocoabuilder.com/archive/cocoa/224795-nsdictionar...