code: void getSingletonNames(Set<String> singletons) {
          for (Map.Entry<String, ConfigurationList<SimpleElement>> entry : configurationMap.entrySet()) {
              ConfigurationList<SimpleElement> list = entry.getValue();
              if (!list.isEmpty() && !list.hasId()) {
                  singletons.add(entry.getKey());
              }
          }
      }
docs: guarantees here, so this should really go away.