title: how should I concatenate variables and strings in ng-repeat
body: <p>I have a list of <code>parents</code>: <code>['a','b','c']</code>, 
  and 3 lists of children: </p>
  
  <pre><code>$ctrl.aChildren = ['aa','aaa']
  $ctrl.bChildren = ['bb','bbb']
  $ctrl.cChildren = ['cc','ccc']
  </code></pre>
  
  <p>I'd like to access these 3 variables in the inner <code>&lt;li&gt;</code> using variable <code>parent</code> from outer <code>&lt;li&gt;</code></p>
  
  <pre><code>&lt;li ng-repeat="parent in $ctrl.parents"&gt;
      ....
      &lt;ul&gt;
          &lt;li ng-repeat="child in $ctrl[{{parent}} + 'Children']"&gt;
          &lt;/li&gt;
      &lt;/ul&gt;
      ...
  &lt;/li&gt;
  </code></pre>
  
  <p>I got an error saying </p>
  
  <blockquote>
    <p>'{' is an unexpected</p>
  </blockquote>
  
  <p>Thanks</p>