class AddFixedCurve implements Curve, ImmutableBean, Serializable {

  @Override
  public double firstDerivative(double x) {
    return fixedCurve.firstDerivative(x) + spreadCurve.firstDerivative(x);
  }

  private  AddFixedCurve(
      Curve fixedCurve,
      Curve spreadCurve);

}