public ActionResult Pay(int gasPumpIndex, int cashRegisterIndex)
        {
            // get selected gas pump and cash register
            GasPump      gasPump      = Context.GetGasStation().GetGasPumps()[gasPumpIndex];
            CashRegister cashRegister = Context.GetGasStation().GetCashRegisters()[cashRegisterIndex];

            // call 'AcceptInput' on selected cash register
            cashRegister.AcceptInput(gasPump.GetPaymentAmount());

            return(RedirectToAction("Index", new { gasPumpIndex }));
        }