code: private Map<String, SortedSet<String>> getNamesPerSurnameMap(
              final String surname) {
          if (!surnameIndex.containsKey(surname)) {
              return Collections.emptyMap();
          }
          return surnameIndex.get(surname);
      }
docs: Get map of full names to sets of IDs for the provided surname.
  
  @param surname the surname to search.
  @return the map