public class AdvancedInvestment: GeneralInvestment {
    let incremental: Dollars
    let period: Years

    init(initialAmount: Dollars, time: Years, tax: Double, incremental: Dollars, period: Years) {
        self.incremental = incremental
        self.period = period
        super.init(initialAmount: initialAmount, time: time, tax: tax)
    }
}

public final class InvestmentManager {

    func calculateReturn(investment: GeneralInvestment) -> Dollars {
        if let advanced = investment as? AdvancedInvestment {
            return advanced.initialAmount + Double(advanced.time) * (advanced.initialAmount * advanced.tax) + (Double(advanced.period) * (advanced.incremental * advanced.tax))
        } else {
            return investment.initialAmount + Double(investment.time) * (investment.initialAmount * investment.tax)
        }
    }

}