docs: public static PeriodType yearDay() {
          PeriodType type = cYD;
          if (type == null) {
              type = new PeriodType(
                  "YearDay",
                  new DurationFieldType[] {
                      DurationFieldType.years(), DurationFieldType.days(),
                  },
                  new int[] { 0, -1, -1, 1, -1, -1, -1, -1, }
              );
              cYD = type;
          }
          return type;
      }