docs: @NotNull
      public String toSimpleString(boolean allowDecimal)
      {
          if (_denominator == 0 && _numerator != 0) {
              return toString();
          } else if (isInteger()) {
              return Integer.toString(intValue());
          } else if (_numerator != 1 && _denominator % _numerator == 0) {
              // common factor between denominator and numerator
              long newDenominator = _denominator / _numerator;
              return new Rational(1, newDenominator).toSimpleString(allowDecimal);
          } else {
              Rational simplifiedInstance = getSimplifiedInstance();
              if (allowDecimal) {
                  String doubleString = Double.toString(simplifiedInstance.doubleValue());
                  if (doubleString.length() < 5) {
                      return doubleString;
                  }
              }
              return simplifiedInstance.toString();
          }
      }