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The reason for this error is that the ''copy'' and ''mutableCopy'' methods inherited from the ''NSObject'' class are trying to call a method called ''''copyWithZone''. Unfortunately we have not yet implmented this object in our BankAccount class. The next step, therefore, is to learn how to write such a class.
== <NSCopying> Protocol and copyWithZone Method Implementation ==
The first step in implementing the <NSCopying> protocol is to declare that the class conforms. This is achieved in the @interface section of the class. For example:
