title: Use previous item in foreach knockout binding
body: <p>I have the following problem: I have an observable array (populated from the server), which looks like this:</p>
  
  <pre><code>this.elems = ko.observableArray([
      {n: 1, t : 'hello'},
      {n: 1, t : 'why'},
      {n: 1, t : 'are'},
      {n: 2, t : 'some'},
      {n: 2, t : 'ducks'},
      {n: 3, t : 'here'},
      {n: 8, t : '?'}
  ]);
  </code></pre>
  
  <p>And I want to show it in the following way:</p>
  
  <pre><code>      1
  Hello
  why
  are
        2
  some
  ducks
        3
  here
        8
  ?
  </code></pre>
  
  <p>As you see the number is shown only if the previous number is not the same as current.
  This would be super easy with the <code>foreach</code> binding (here is my <a href="http://jsfiddle.net/62Fz8/" rel="nofollow">jsFiddle</a>). </p>
  
  <pre><code>&lt;div data-bind="foreach: elems"&gt;
      &lt;div class="t1"&gt;
          &lt;span data-bind="text: n" class="c1"&gt;&lt;/span&gt;
          &lt;span data-bind="text: t" class="c2"&gt;&lt;/span&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Only if I would be able to access previous the previous element. 
  I know that I can modify my observalbe array and achieve what I want, but is there a way to do this without changing it?</p>