code: public Set<String> getNamesPerSurname(final String surname) {
          if (surname == null) {
              return Collections.emptySet();
          }
          if (!surnameIndex.containsKey(surname)) {
              return Collections.emptySet();
          }
          return Collections.unmodifiableSet(surnameIndex.get(surname).keySet());
      }
docs: Get the set of full names that occur for a given surname.
  
  @param surname the surname to search.
  @return the keys