title: Python itertools.combinations: how to obtain the indices of the combined numbers
body: <p>The result created by Python's itertools.combinations() is the combinations of numbers. For example:</p>
  
  <pre><code>a = [7, 5, 5, 4]
  b = list(itertools.combinations(a, 2))
  
  # b = [(7, 5), (7, 5), (7, 4), (5, 5), (5, 4), (5, 4)]
  </code></pre>
  
  <p>But I would like to also obtain the indices of the combinations such as:</p>
  
  <pre><code>index = [(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)]
  </code></pre>
  
  <p>How can I do it?</p>