title: NumPy ndarray slicing and indexing using comma
body: <p>What basically the commas do in slicing nd array in NumPy?</p>
  <p>E.g. for a 3d array, what do these mean?
  <code>a[1,2::] a[:2,3:] a[1:2,:]</code></p>
  <p>I means commas in general, how do these thing work while slicing? I know about
  <code>a[start:stop:step]</code> but having hard time understanding what to commas imply.</p>
  <p>Edit: I've read on few websites about slicing using tuple,that is what I don't understand. How does slicing with tuple work?</p>