docs: public static final ResourceKey plain(String key) {
  		int idx = key.lastIndexOf(BUNDLE_ID_SEPARATOR);
  		if (idx < 0) {
  			return new ResourceKey(null, key);
  		}
  		String bundle = idx > 0 ? key.substring(0, idx) : null;
  		String id = idx < key.length() - 1 ? key.substring(idx + 1) : null;
  		return key(bundle, id);
  	}