docs: private static boolean _isValidForCycle (@Nonnull final Holiday aHoliday, final int nYear)
    {
      final String sEvery = aHoliday.getEvery ();
      if (sEvery != null && !"EVERY_YEAR".equals (sEvery))
      {
        if ("ODD_YEARS".equals (sEvery))
          return nYear % 2 != 0;
        if ("EVEN_YEARS".equals (sEvery))
          return nYear % 2 == 0;
  
        if (aHoliday.getValidFrom () != null)
        {
          int nCycleYears = 0;
          if ("2_YEARS".equalsIgnoreCase (sEvery))
            nCycleYears = 2;
          else
            if ("3_YEARS".equalsIgnoreCase (sEvery))
              nCycleYears = 3;
            else
              if ("4_YEARS".equalsIgnoreCase (sEvery))
                nCycleYears = 4;
              else
                if ("5_YEARS".equalsIgnoreCase (sEvery))
                  nCycleYears = 5;
                else
                  if ("6_YEARS".equalsIgnoreCase (sEvery))
                    nCycleYears = 6;
                  else
                    throw new IllegalArgumentException ("Cannot handle unknown cycle type '" + sEvery + "'.");
  
          return (nYear - aHoliday.getValidFrom ().intValue ()) % nCycleYears == 0;
        }
      }
      return true;
    }