code: @Override
      public boolean isLeapYear(long prolepticYear) {
          long lastTwoDigits = prolepticYear % 100;
          return Math.abs(lastTwoDigits) == 99 || (prolepticYear % 400 != 0 && (lastTwoDigits == 0 || lastTwoDigits % 6 == 0));
      }
docs: Checks if the specified year is a leap year.
  <p>
  In general, a year is a leap year if the last two digits are divisible by 6 without remainder, or are 99.
  Years with the last two digits of 00 are also leap years, with the exception of years divisible by 400, which are not.
  <p>
  For example, 2012 is a leap year because the last two digits (12) are divisible by 6.
  1999 is a leap year as the last two digits are both 9's (99).
  1900 is a leap year as the last two digits are both 0's (00), however 2000 was not a leap year as it is divisible by 400.
  The year 0 is not a leap year.
  
  @param prolepticYear  the proleptic-year to check, not validated for range
  @return true if the year is a leap year