docs: public static Set<SelectorName> nameSetFrom( Set<SelectorName> firstSet,
                                                   Set<SelectorName> secondSet ) {
          if ((firstSet == null || firstSet.isEmpty()) && (secondSet == null || secondSet.isEmpty())) {
              return Collections.emptySet();
          }
          Set<SelectorName> result = new LinkedHashSet<SelectorName>();
          result.addAll(firstSet);
          if (secondSet != null) result.addAll(secondSet);
          return Collections.unmodifiableSet(result);
      }