title: defaultdict of defaultdict?
body: <p>Is there a way to have a <code>defaultdict(defaultdict(int))</code> in order to make the following code work?</p>
  
  <pre><code>for x in stuff:
      d[x.a][x.b] += x.c_int
  </code></pre>
  
  <p><code>d</code> needs to be built ad-hoc, depending on <code>x.a</code> and <code>x.b</code> elements.</p>
  
  <p>I could use:</p>
  
  <pre><code>for x in stuff:
      d[x.a,x.b] += x.c_int
  </code></pre>
  
  <p>but then I wouldn't be able to use:</p>
  
  <pre><code>d.keys()
  d[x.a].keys()
  </code></pre>