code: public void setKappa(double kappa)
      {
          if(kappa < 0.5 || kappa > 1.0 || Double.isNaN(kappa))
              throw new IllegalArgumentException("Kapp must be in [0.5, 1], not " + kappa);
          this.kappa = kappa;
      }
docs: The "forgetfulness" factor in the learning rate. Larger values increase
  the rate at which old information is "forgotten"
  @param kappa the forgetfulness factor in [0.5, 1]