title: Conditional 'click' binding with Knockout
body: <p>I am binding the click event to every list item in a list: </p>
  
  <pre><code>        &lt;ul class="modal-subject-list" data-bind="foreach: filteredSubjects"&gt;
              &lt;li data-bind="click: $parent.pickSubject, css: {alreadyAddedBackground: hasBeenAdded}"&gt;
                  &lt;!-- Lots of code here --&gt;
              &lt;/li&gt;
          &lt;/ul&gt;
  </code></pre>
  
  <p>I want to disable the <code>click:</code> binding if 'hasBeenAdded' resolves to true.  I know some messy ways to take care of it:</p>
  
  <ol>
  <li>Have two list items, one for <code>if: hasBeenAdded</code>, and the other for <code>if: !hasBeenAdded</code>.  This is far from DRY</li>
  <li>Handle this check with javascript and leave the view alone - I don't like this because unnecessary markup is being generated for list items that should be essentially disabled.</li>
  </ol>
  
  <p>Is there a way to register a "clickIf" binding?</p>