// Instrument interface
        public override void calculate()
        {
            Utils.QL_REQUIRE(!discountCurve_.empty(), () => "discounting term structure handle is empty");

            results_.value         = results_.cash = 0;
            results_.errorEstimate = null;

            Date refDate = discountCurve_.link.referenceDate();

            Date settlementDate = settlementDate_;

            if (settlementDate_ == null)
            {
                settlementDate = refDate;
            }
            else
            {
                Utils.QL_REQUIRE(settlementDate >= refDate, () =>
                                 "settlement date (" + settlementDate + ") before " +
                                 "discount curve reference date (" + refDate + ")");
            }

            results_.valuationDate = npvDate_;
            if (npvDate_ == null)
            {
                results_.valuationDate = refDate;
            }
            else
            {
                Utils.QL_REQUIRE(npvDate_ >= refDate, () =>
                                 "npv date (" + npvDate_ + ") before " +
                                 "discount curve reference date (" + refDate + ")");
            }

            results_.npvDateDiscount = discountCurve_.link.discount(results_.valuationDate);

            int n = arguments_.legs.Count;

            results_.legNPV         = new InitializedList <double?>(n);
            results_.legBPS         = new InitializedList <double?>(n);
            results_.startDiscounts = new InitializedList <double?>(n);
            results_.endDiscounts   = new InitializedList <double?>(n);

            bool includeRefDateFlows =
                includeSettlementDateFlows_.HasValue ?
                includeSettlementDateFlows_.Value :
                Settings.Instance.includeReferenceDateEvents;

            for (int i = 0; i < n; ++i)
            {
                try
                {
                    YieldTermStructure discount_ref = discountCurve_.currentLink();
                    double             npv = 0, bps = 0;
                    CashFlows.npvbps(arguments_.legs[i],
                                     discount_ref,
                                     includeRefDateFlows,
                                     settlementDate,
                                     results_.valuationDate,
                                     out npv,
                                     out bps);
                    results_.legNPV[i] = npv * arguments_.payer[i];
                    results_.legBPS[i] = bps * arguments_.payer[i];

                    if (!arguments_.legs[i].empty())
                    {
                        Date d1 = CashFlows.startDate(arguments_.legs[i]);
                        if (d1 >= refDate)
                        {
                            results_.startDiscounts[i] = discountCurve_.link.discount(d1);
                        }
                        else
                        {
                            results_.startDiscounts[i] = null;
                        }

                        Date d2 = CashFlows.maturityDate(arguments_.legs[i]);
                        if (d2 >= refDate)
                        {
                            results_.endDiscounts[i] = discountCurve_.link.discount(d2);
                        }
                        else
                        {
                            results_.endDiscounts[i] = null;
                        }
                    }
                    else
                    {
                        results_.startDiscounts[i] = null;
                        results_.endDiscounts[i]   = null;
                    }
                }
                catch (Exception e)
                {
                    Utils.QL_FAIL((i + 1) + " leg: " + e.Message);
                }
                results_.value += results_.legNPV[i];
            }
        }