code: public void setXi(double xi)
      {
          if(xi <= 0 || xi >= 1 || Double.isNaN(xi))
              throw new ArithmeticException("xi must be in the range (0, 1) not " + xi);
          this.xi = xi;
          this.one_min_xi = 1.0 - xi;
      }
docs: Sets the xi value used in {@link ExtractionMethod#XI_STEEP_ORIGINAL} to
  produce cluster results.
  
  @param xi the value in the range (0, 1)
  @throws ArithmeticException if the value is not in the appropriate range