This protocol declares the single method mutableCopyWithZone:, which is implemented by classes that wish to allow clients to make mutable copies of themselves. This protocol should be adopted only by classes that have both mutable and immutable variants (for example, NSString and NSMutableString). Clients create mutable copies of objects through NSObject's mutableCopy convenience method, which invokes mutableCopyWithZone: with the default zone.

@protocol NSMutableCopying
// Instance Methods
- (id)mutableCopyWithZone:(NSZone *)zone;
@end