docs: public PeriodConverter addPeriodConverter(PeriodConverter converter)
              throws SecurityException {
          
          checkAlterPeriodConverters();
          if (converter == null) {
              return null;
          }
          PeriodConverter[] removed = new PeriodConverter[1];
          iPeriodConverters = iPeriodConverters.add(converter, removed);
          return removed[0];
      }