Change the Readable protocol and the ReadableObject and ReadOnceObject classes, so that:

ReadableObject and ReadOnceObject implement the Readable protocol.
When the read method is called on the ReadableObject class, it returns the value that it received in the constructor.
When the read method is called on the ReadOnceObject class it returns the value that it received in the constructor the first time it’s called and returns an empty string every other time.
The isReadable method returns true in the ReadableObject class. The isReadable method of the ReadOnceObject class should return true the first time read is called, and false every other time.