code: 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);
  	}
docs: Constructs new resource key from plain string according to following rules:<ul>
  <li>Last dot in the string separates key bundle from key id.</li>
  <li>Strings starting from the dot represent a key with <code>null</code> bundle (default bundle).</li>
  <li>Strings ending with a dot represent keys with <code>null</code> id (bundles).</li>
  </ul>
  This operation is inversion of {@link #toString()} method for identifiers
  not containing {@link #ID_COMPONENT_DELIMITER} character:<ul>
  <li>string.equals(plain(string).toString())</li>
  <li>key.equals(plain(key.toString()))</li>
  </ul>
  @param key string representation of a key
  @return the key corresponding to given string