public HybridHestonHullWhiteProcess(HestonProcess hestonProcess,
                                            HullWhiteForwardProcess hullWhiteProcess,
                                            double corrEquityShortRate,
                                            Discretization discretization = Discretization.BSMHullWhite)
        {
            hestonProcess_    = hestonProcess;
            hullWhiteProcess_ = hullWhiteProcess;
            hullWhiteModel_   = new HullWhite(hestonProcess.riskFreeRate(),
                                              hullWhiteProcess.a(),
                                              hullWhiteProcess.sigma());
            corrEquityShortRate_ = corrEquityShortRate;
            disc_   = discretization;
            maxRho_ = Math.Sqrt(1 - hestonProcess.rho() * hestonProcess.rho())
                      - Math.Sqrt(Const.QL_EPSILON) /* reserve for rounding errors */;

            T_           = hullWhiteProcess.getForwardMeasureTime();
            endDiscount_ = hestonProcess.riskFreeRate().link.discount(T_);

            Utils.QL_REQUIRE(corrEquityShortRate * corrEquityShortRate
                             + hestonProcess.rho() * hestonProcess.rho() <= 1.0, () =>
                             "correlation matrix is not positive definite");

            Utils.QL_REQUIRE(hullWhiteProcess.sigma() > 0.0, () =>
                             "positive vol of Hull White process is required");
        }
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        protected override IPricingEngine controlPricingEngine()
        {
            HybridHestonHullWhiteProcess process = process_ as HybridHestonHullWhiteProcess;

            Utils.QL_REQUIRE(process != null, () => "invalid process");

            HestonProcess hestonProcess = process.hestonProcess();

            HullWhiteForwardProcess hullWhiteProcess = process.hullWhiteProcess();

            HestonModel hestonModel = new HestonModel(hestonProcess);

            HullWhite hwModel = new HullWhite(hestonProcess.riskFreeRate(),
                                              hullWhiteProcess.a(),
                                              hullWhiteProcess.sigma());

            return(new AnalyticHestonHullWhiteEngine(hestonModel, hwModel, 144));
        }