'weak' should not be applied to a property declaration

[kumapo]

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[kumapo]

• https://docs.swift.org/swift-book/LanguageGuide/Protocols.html

  Note that the delegate property is defined as an optional DiceGameDelegate, because a delegate isn’t required in order to play the game. Because it’s of an optional type, the delegate property is automatically set to an initial value of nil. Thereafter, the game instantiator has the option to set the property to a suitable delegate. Because the DiceGameDelegate protocol is class-only, you can declare the delegate to be weak to prevent reference cycles.