code: public void setAlpha(double alpha)
      {
          if(alpha < 0 || Double.isNaN(alpha) || Double.isInfinite(alpha))
              throw new IllegalArgumentException("Extra parameter must be non negative, not " + alpha);
          this.alpha = alpha;
      }
docs: Sets the extra parameter alpha. This is used for some priors that take
  an extra parameter. This is {@link Prior#CAUCHY} and
  {@link Prior#ELASTIC}. If these two priors are not in use, the value is
  ignored.
  
  @param alpha the extra parameter value to use. Must be positive