code: private SortedMap<String, SortedSet<String>> findNamesPerSurname(
              final String surname) {
          if (surnameIndex.containsKey(surname)) {
              return surnameIndex.get(surname);
          }
          final SortedMap<String, SortedSet<String>> namesPerSurname =
                  new TreeMap<String, SortedSet<String>>();
          surnameIndex.put(surname, namesPerSurname);
          return namesPerSurname;
      }
docs: Find the map of full names to a sets of IDs for the given surname. If the
  surname is not already present, create a new map and add it to the index.
  
  @param surname surname to search
  @return the associated map