list a in general layout schema elements

Author: davidcarlisle

src/presentation-markup.html

@@ -821,7 +821,10 @@ <h5 id="presm_general_layout">General Layout Schemata</h5>
      <tbody>
 
       <tr>
-       <td><a class="intref" href="#presm_mrow"><code class="element">mrow</code></a></td>
+	<td>
+	  <a class="intref" href="#presm_mrow"><code class="element">mrow</code></a>
+	  and
+	  <a class="intref" href="#presm_mrow"><code class="element">a</code></a></td>
        <td>group any number of sub-expressions horizontally</td>
       </tr>