protected override double zeroYieldImpl(double t)
{
// to be fixed: user-defined daycounter should be used
InterestRate zeroRate = originalCurve_.link.zeroRate(t, comp_, freq_, true);
InterestRate spreadedRate = new InterestRate(zeroRate.value() + spread_.value(), zeroRate.dayCounter(),
zeroRate.compounding(), zeroRate.frequency());
return(spreadedRate.equivalentRate(t, Compounding.Continuous, Frequency.NoFrequency).value());
}