code: public Deque<Subfield> getSubfield(String subfieldId) {
          return subfields.stream()
                  .filter(subfield -> subfield.getId().equals(subfieldId))
                  .collect(Collectors.toCollection(LinkedList::new));
      }
docs: Return all subfields of a given subfield ID. Multiple occurences may occur.
  @param subfieldId subfield ID
  @return list of subfields