Concurrent inputs - Added example from 840

understanding/21/concurrent-input-mechanisms.html

@@ -29,7 +29,7 @@ <h2>Examples</h2>
         <li>On a touch-enabled laptop with coarse precision, people who have difficulty activating a small target because of hand tremors, limited dexterity or other reasons are still able to interact with content using their keyboard and trackpad.</li>
         <li>A user starts interacting with a page using a desktop keyboard, and then attaches a secondary touch-enabled monitor. Content can be operated using this newly added input mechanism and does not assume that the keyboard, the first input mechanism it detected, is the only one in use.</li>
         <li>A speech input user navigates content using voice commands which translate to simulate mouse (and keyboard) commands. When talking with a colleague, however, the user turns speech recognition off and uses the mouse instead.</li>
-
+        <li>A user opens a menu with a mouse, and then navigates between the menu items with arrow keys.</li>
       </ul>
 		</section>
 		<section id="resources">