docs: public Rational Pochhammer(final BigInteger n) {
          if (n.compareTo(BigInteger.ZERO) < 0) {
              return null;
          } else if (n.compareTo(BigInteger.ZERO) == 0) {
              return Rational.ONE;
          } else {
              /* initialize results with the current value
               */
              Rational res = new Rational(a, b);
              BigInteger i = BigInteger.ONE;
              for (; i.compareTo(n) < 0; i = i.add(BigInteger.ONE)) {
                  res = res.multiply(add(i));
              }
              return res;
          }
      }