title: numpy array indexing with negative index
body: <p>First I have a scalar time series stored in a numpy array:</p>
  
  <pre><code>ts = np.arange(10)
  </code></pre>
  
  <p>which is</p>
  
  <pre><code>array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
  </code></pre>
  
  <p>Suppose I want to extract from <code>ts</code> a series of vectors <code>(2,1,0)</code>, <code>(3,2,1)</code>, <code>(4,3,2)</code>, etc., I can think of the following code to do it:</p>
  
  <pre><code>for i in range(len(ts)-2):
      print(ts[2+i:i-1:-1])
  </code></pre>
  
  <p>However, when <code>i=0</code>, the above code returns an empty array rather than <code>[2,1,0]</code> because the loop body will become</p>
  
  <pre><code>print(ts[2:-1:-1])
  </code></pre>
  
  <p>where the -1 in the middle creates trouble.</p>
  
  <p><strong>My question is</strong>: is there a way to make the indexing work for <code>[2,1,0]</code>?</p>