code: @Override
      public Collection<String> findSurnameInitialLetters(
              final FinderObject owner) {
          final Root root = (Root) owner;
          final RootVisitor visitor = new RootVisitor();
          root.accept(visitor);
          final Set<String> matches = new TreeSet<>();
          for (final Person person : visitor.getPersons()) {
              final String firstLetter = person.getSurname().substring(0, 1);
              matches.add(firstLetter);
          }
          return matches;
      }
docs: {@inheritDoc}