code: public void setEta(double eta)
      {
          if(eta <= 0 || Double.isInfinite(eta) || Double.isNaN(eta))
              throw new IllegalArgumentException("Eta must be a positive constant, not " + eta);
          this.eta = eta;
      }
docs: Prior on topics. 1/{@link #setK(int) K} is  a common choice.
  @param eta the positive prior for topics