title: numpy: referencing elements in reverse order
body: <p>I am a bit confused on how to reference elements of a numpy array in reverse order: For example:</p>
  
  <pre><code>arr = np.array([[1,2,3,4,5],[6,7,8,9,10],[11,12,13,14,15],[16,17,18,19,20]])
  print(arr[1:3,:]) #gives me
  [[ 6  7  8  9 10]
   [11 12 13 14 15]]
  </code></pre>
  
  <p>However </p>
  
  <pre><code>print(arr[3:1,:]  #gives me
  []
  </code></pre>
  
  <p>Is there anyway I can start at element 3 and print backwards till element 1?
  So expected output is</p>
  
  <pre><code>[[11 12 13 14 15]
   [ 6  7  8  9 10]]
  </code></pre>
  
  <p>Thanks!</p>