using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

}

​

public class Payment { }

​

public interface IPaymentGateway

{

    IReadOnlyCollection<Payment> GetPayments(int userId);

}

​

public class PaymentGateway : IPaymentGateway

{

    public IReadOnlyCollection<Payment> GetPayments(int userId)

    {

        throw new NotImplementedException("There will be a unreliable network call.");

    }

}

​

public class ReliablePaymentGateway : IPaymentGateway

{

    public IReadOnlyCollection<Payment> GetPayments(int userId)

    {

        //TODO: Implement!

        throw new NotImplementedException();

    }

}