title: how to get a defautdict(set) like this form : {str : { str : int } }
body: <p>I got a  dictionary like this</p>
  
  <pre><code>{('a','b'):3, ('b','c'): 2, ('a','c'): 5}
  </code></pre>
  
  <p>I want to convert it to this form :</p>
  
  <pre><code>{'b': {'c': 2}, 'a': {'c': 5, 'b': 3}}  
  </code></pre>
  
  <p>That means I have to build a dictionary like this form:</p>
  
  <pre><code>   d:{(str1,str2):int}   --&gt;     {str1 : { str2 : int } }
  </code></pre>