code: @Override
      public Collection<String> findBySurnamesBeginWith(final FinderObject owner,
              final String beginsWith) {
          logger.info("Starting findBySurnamesBeginWith");
          final Set<String> surnames = new TreeSet<>();
          if (owner instanceof Root) {
              final Root root = (Root) owner;
              final RootDocumentMongo rootDocument =
                      (RootDocumentMongo) toDocConverter.createGedDocument(root);
              final Collection<PersonDocument> personDocuments =
                      repositoryManager.getPersonDocumentRepository()
                      .findByRootAndSurnameBeginsWith(rootDocument, beginsWith);
              for (final PersonDocument personDocument : personDocuments) {
                  surnames.add(personDocument.getSurname());
              }
          }
          logger.info("Ending findBySurnamesBeginWith");
          return surnames;
      }
docs: {@inheritDoc}