title: Angular.js. Ng-model of checkbox
body: <p>Here's my code:</p>
  
  <pre><code>&lt;input type="checkbox" ng-model="variable"&gt;
  </code></pre>
  
  <p>In this case <code>variable</code> will be <code>true</code> or <code>false</code> according the status of checkbox. I need to set not <code>true</code>/<code>false</code> but <code>"+"</code>/<code>"-"</code> instead. What is the simplest way to do this?
  Thanks.</p>