/*! \pre the \f$ x \f$ values must be sorted. */
 public BackwardFlatInterpolation(List <double> xBegin, int size, List <double> yBegin)
 {
     impl_ = new BackwardFlatInterpolationImpl(xBegin, size, yBegin);
     impl_.update();
 }
 /*! \pre the \f$ x \f$ values must be sorted. */
 public BackwardFlatInterpolation(List<double> xBegin, int size, List<double> yBegin) {
     impl_ = new BackwardFlatInterpolationImpl(xBegin, size, yBegin);
     impl_.update();
 }