code: @Override
      public KitCalculatorsFactory<E> registerHolidays(final String name, final HolidayCalendar<E> holidaysCalendar) {
          if (name != null) {
              final Set<E> hol = new HashSet<>();
              if (holidaysCalendar != null && holidaysCalendar.getHolidays() != null) {
                  hol.addAll(holidaysCalendar.getHolidays());
              }
              final DefaultHolidayCalendar<E> defaultHolidayCalendar = new DefaultHolidayCalendar<>(hol);
              if (holidaysCalendar != null) {
                  defaultHolidayCalendar.setEarlyBoundary(holidaysCalendar.getEarlyBoundary());
                  defaultHolidayCalendar.setLateBoundary(holidaysCalendar.getLateBoundary());
              }
              this.holidays.put(name, new ImmutableHolidayCalendar<E>(holidaysCalendar));
          }
          return this;
      }
docs: Use this method to register a given calendar, it will replace any
  existing one with the same name. An immutable copy is made so that any changes outside this class
  will have no affect. It won't update any existing DateCalculator as these should
  not be amended whilst in existence (we could otherwise get inconsistent
  results).
  
  @param name
  the calendar name to register these holidays under.
  @param holidaysCalendar
  a calendar containing a set of holidays (non-working days).