code: @Override
      public YearMonth plus(long amountToAdd, TemporalUnit unit) {
          if (unit instanceof ChronoUnit) {
              switch ((ChronoUnit) unit) {
                  case MONTHS: return plusMonths(amountToAdd);
                  case YEARS: return plusYears(amountToAdd);
                  case DECADES: return plusYears(Jdk8Methods.safeMultiply(amountToAdd, 10));
                  case CENTURIES: return plusYears(Jdk8Methods.safeMultiply(amountToAdd, 100));
                  case MILLENNIA: return plusYears(Jdk8Methods.safeMultiply(amountToAdd, 1000));
                  case ERAS: return with(ERA, Jdk8Methods.safeAdd(getLong(ERA), amountToAdd));
              }
              throw new UnsupportedTemporalTypeException("Unsupported unit: " + unit);
          }
          return unit.addTo(this, amountToAdd);
      }
docs: {@inheritDoc}
  @throws DateTimeException {@inheritDoc}
  @throws ArithmeticException {@inheritDoc}