title: defaultdict tuple of lists
body: <p>I like defaultdict, but I want it to autovivify a 2-tuple of lists and I'm not sure if it's possible.  So what I want is:</p>
  
  <pre><code>foo = defaultdict(???)
  foo['key1'][0].append('value')
  foo['key1'][1].append('other value')
  </code></pre>
  
  <p>is this do-able with defaultdict?</p>