title: Add an extra value to a dict key
body: <p>If have to following dict:</p>
  
  <pre><code>graph = {'A': ['B', 'C'], 'B': ['C', 'D']}
  </code></pre>
  
  <p>How can i add an extra value to (for example)  key a (so the result will be </p>
  
  <pre><code>graph = {'A': ['B', 'C', 'Z'], 'B': ['C', 'D']}
  </code></pre>