code: public final PeriodList add(final PeriodList periods) {
          if (periods != null) {
              final PeriodList newList = new PeriodList();
              newList.addAll(this);
              newList.addAll(periods);
              return newList.normalise();
          }
          return this;
      }
docs: A convenience method that combines all the periods in the specified list to
  this list. The result returned is a new PeriodList instance, except where
  no periods are specified in the arguments. In such cases this instance is returned.
  
  Normalisation is also performed automatically after all periods have been added.
  
  @param periods a list of periods to add
  @return a period list instance