title: knockout data-bind with $index poses problems
body: <p>Concider a jQuery Mobile "list" containing a radio button and a label for it.</p>
  
  <pre><code>&lt;!-- ko foreach: $data.answers --&gt;
  &lt;input type="radio" name="radio-choice" data-bind="attr: { id: [...] }" /&gt;
  &lt;label data-bind="attr:{ for: [...] }"&gt;Label&lt;/label&gt;
  &lt;!-- /ko --&gt;
  </code></pre>
  
  <p>In order to work, the <strong>for</strong> attribute of the label needs to be the same as the <strong>id</strong> of the  input.</p>
  
  <pre><code>REPLACEMENT FOR [...]            RESULTS IN
  $index                           ok
  'radio-nr-'+$index               fails
  $root.testFunction(1)            ok
  $root.testFunction($index)       fails
  'radio-nr-'.concat(1)            ok
  'radio-nr-'.concat($index)       fails
  </code></pre>
  
  <p>where</p>
  
  <pre><code>function testFunction(a) {  return "radio-nr-"+a; };
  </code></pre>
  
  <p>Why do all my attempts of concatenating the $index fail?</p>
  
  <p>Thanks!</p>