title: Python Numpy syntax: what does array index as two arrays separated by comma mean?
body: <p>I don't understand array as index in Python Numpy.
  For example, I have a 2d array A in Numpy</p>
  
  <pre><code>[[1,2,3]
   [4,5,6]
   [7,8,9]
   [10,11,12]]
  </code></pre>
  
  <p>What does A[[1,3], [0,1]] mean?</p>