code: public void setExponent(double expo)
      {
          if(expo <= 0 || Double.isInfinite(expo) || Double.isNaN(expo))
              throw new ArithmeticException("The exponent must be a positive constant");
          this.expo = expo;
      }
docs: Weights are updated as 1+coef*errors<sup>expo</sup>. This sets the
  exponent used to update the errors
  @param expo the exponent to use