let five = ArithmeticExpression.number(5)
let four = ArithmeticExpression.number(4)
let sum = ArithmeticExpression.addition(five, four)
let product = ArithmeticExpression.multiplication(sum, ArithmeticExpression.number(2))

func eva luate(_ expression: ArithmeticExpression) -> Int {
    switch expression {
    case let .number(value):
        return value
    case let .addition(left, right):
        return eva luate(left) + eva luate(right)
    case let .multiplication(left, right):
        return eva luate(left) * eva luate(right)
    }
}
 
print(eva luate(product))
// Prints "18"