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^

<stdin>:25:14: error: var overrides a 'final' var

override var radius: Double {

^

<stdin>:6:14: note: overridden declaration is here

final var radius = 12.5

Since the super class is declared as 'final' and its data types are also declared as 'final' the
program won't allow to create subclasses further and it will throw errors.