code: public List<MarcField> filterKey(Pattern pattern) {
          return marcFields.stream()
                  .map(field -> field.matchKey(pattern))
                  .filter(Objects::nonNull)
                  .collect(Collectors.toList());
      }
docs: Return a list of MARC fields of this record where key pattern matches were found.
  
  @param pattern the pattern
  @return a list of MARC fields