Objects that implement the NSMutableCopying protocol will a new instance of the mutable version of the object. For example calling MutableCopy on an NSCharacterSet returns an NSMutableCharacterSet.
Types that implement the NSMutableCopying protocol include: AVFoundation.AVAudioMix, AVFoundation.AVAudioMixInputParameters, AVFoundation.AVComposition, AVFoundation.AVMetadataItem, AVFoundation.AVVideoComposition, AVFoundation.AVVideoCompositionInstruction, AVFoundation.AVVideoCompositionLayerInstruction, Foundation.NSArray, Foundation.NSAttributedString, Foundation.NSCharacterSet, Foundation.NSData, Foundation.NSDictionary, Foundation.NSIndexSet, Foundation.NSObject, Foundation.NSSet, Foundation.NSString, Foundation.NSUrlRequest, StoreKit.SKPayment and UIKit.NSParagraphStyle.