title: List of lists of tuples, sum element-wise
body: <p>I have a list of lists of tuples. Each inner list contains 3 tuples, of 2 elements each:</p>
  <pre><code>[
  [(3, 5), (4, 5), (4, 5)],
  [(7, 13), (9, 13), (10, 13)],
  [(5, 7), (6, 7), (7, 7)]
  ]
  </code></pre>
  <p>I need to get a single list of 3 tuples, summing all these elements &quot;vertically&quot;, like this:</p>
  <pre><code>(3, 5),  (4, 5),  (4, 5)
   +  +     +  +     +  +  
  (7, 13), (9, 13), (10, 13)
   +  +     +  +     +  +  
  (5, 7),  (6, 7),  (7, 7)
    ||       ||       ||
  [(15, 25), (19, 25), (21, 25)]
  </code></pre>
  <p>so, for example, the second tuple in the result list is given by the sums of the second tuples in the initial list</p>
  <p>(4+9+6, 5+13+7) = (19, 25)</p>
  <p>I'm trying with list/tuple comprehensions, but I'm getting a little lost with this.</p>