title: Xpath select all child nodes
body: <p>I need select all child nodes (option tag) from this html:</p>
  
  <pre><code>&lt;select name="akt-miest" id="onoffaci"&gt;
  &lt;option value="a_0"&gt;Všetci&lt;/option&gt;
  &lt;option value="a_1"&gt;Iba prihlásení&lt;/option&gt;
  &lt;option value="a_5" selected="selected"&gt;Teraz na Pokeci&lt;/option&gt;
  
  &lt;optgroup label="Hlavné miestnosti"&gt;
  
  &lt;option value="m_13"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;Bez záväzkov&lt;/option&gt;
  &lt;option value="m_9"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;Do pohody&lt;/option&gt;
  &lt;option value="m_39"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;Dámsky klub&lt;/option&gt;
  
  &lt;/optgroup&gt;
  </code></pre>
  
  <p>I use Html agility pack.</p>
  
  <p>I try this:</p>
  
  <pre><code> var selectNode = htmlDoc.GetElementbyId("onoffaci");
  
   var nodes = selectNode.SelectNodes("option::*");
  </code></pre>
  
  <p>but I get error that xpath has invalid token. What is bad?</p>
  
  <p>For example:</p>
  
  <pre><code>&lt;option value="**a_0**"&gt;**Všetci**&lt;/option&gt;
  </code></pre>
  
  <p>I need get value (a_0) and text Všetci.</p>
  
  <p>So I try first access to select by Id:</p>