title: how to slice a subset of numpy arrays
body: <p>Given this array:</p>
  
  <pre><code>&gt;&gt;&gt; a
  array([[ 0,  1,  2],
         [ 3,  4,  5],
         [ 6,  7,  8],
         [ 9, 10, 11]])
  </code></pre>
  
  <p>How can I select <code>[[4,5], [7,8]]</code>? <code>a[0::2, 1:;2]</code> doesn't work</p>