final double t2 = definition.getUnderlyingOption().getTimeToExpiry(date);
final double deltaT = t2 - t1;
final double sigma = data.getVolatility(t1, k1); //REVIEW emcleod 20-7-10 This will work with a flat volatility surface but otherwise will give odd results
final double r = data.getInterestRate(t1);
final double b = data.getCostOfCarry();
final double criticalValue = getCriticalValue(new EuropeanVanillaOptionDefinition(k2, new Expiry(DateUtils.getDateOffsetWithYearFraction(date, deltaT)), underlying.isCall()), data, k1);
final double d1 = getD1(s, criticalValue, t1, sigma, b);
final double d2 = getD2(d1, sigma, t1);
final double d3 = getD1(s, k2, t2, sigma, b);
final double d4 = getD2(d3, sigma, t2);
if (definition.isCall()) {