title: Colon, None, slice(None) in numpy array indexers
body: <p>As for <code>a = np.arange(24).reshape(2,3,4)</code></p>
  
  <p><code>a[0,:,1]</code> or <code>a[0,slice(None),1]</code> outputs <code>array([1, 5, 9])</code></p>
  
  <p>while <code>a[0,None,1]</code> gives <code>array([[4, 5, 6, 7]])</code></p>
  
  <p>Could sb explain the latter?</p>