docs: public static double getDeviancePart(double x, double mu) {
          double ret;
          if (FastMath.abs(x - mu) < 0.1 * (x + mu)) {
              double d = x - mu;
              double v = d / (x + mu);
              double s1 = v * d;
              double s = Double.NaN;
              double ej = 2.0 * x * v;
              v = v * v;
              int j = 1;
              while (s1 != s) {
                  s = s1;
                  ej *= v;
                  s1 = s + ej / ((j * 2) + 1);
                  ++j;
              }
              ret = s1;
          } else {
              ret = x * FastMath.log(x / mu) + mu - x;
          }
          return ret;
      }