title: Generalizing adding nested lists
body: <p>Given 3 nested vectors:</p>
  
  <pre><code> &gt;&gt;&gt; a
  [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
  &gt;&gt;&gt; b
  [[10, 20, 30], [40, 50, 60], [70, 80, 90]]
  &gt;&gt;&gt; c
  [[100, 200, 300], [400, 500, 600], [700, 800, 900]]
  </code></pre>
  
  <p>I can add these vectors together with a map/sum/zip comprehension like so:</p>
  
  <pre><code>&gt;&gt;&gt; [map(sum,zip(i,j,k)) for i,j,k in zip(a,b,c)]
  [[111, 222, 333], [444, 555, 666], [777, 888, 999]]
  </code></pre>
  
  <p>I've manually expanded this from adding two lists together, but is there a pythonic way to generalize this to handle an arbitrary number of lists?</p>
  
  <p>(Python 2.7 without using external libraries preferred)</p>