docs: static boolean isRegistered(final Object lhs, final Object rhs) {
          final Set<Pair<IDKey, IDKey>> registry = getRegistry();
          final Pair<IDKey, IDKey> pair = getRegisterPair(lhs, rhs);
          final Pair<IDKey, IDKey> swappedPair = new Pair<IDKey, IDKey>(pair.getKey(), pair.getValue());
  
          return registry != null
                  && (registry.contains(pair) || registry.contains(swappedPair));
      }