title: How np.array (or vstack) column slice work?
body: <p>Why in:</p>
  
  <pre><code>import numpy as np
  a = [1, 2 ,3]
  b = [4, 5, 6]
  t= np.array(([a,b])
  print(t[1:])
  print(t[:0])
  print(t[:1])
  print(t[:2])
  </code></pre>
  
  <p>the result of the first column and second column is :</p>
  
  <pre><code>[[4 5 6]]
  []
  [[1 2 3]]
  [[1 2 3]
   [4 5 6]]
  </code></pre>
  
  <p>If the array is:</p>
  
  <pre><code>[[1 2 3]
   [4 5 6]]
  </code></pre>
  
  <p>Why then the:</p>
  
  <ul>
  <li>first row <code>t[1:]</code> is not <code>[4 5 6]</code> and</li>
  <li>first column <code>t[:0]</code> is not <code>[1, 4]</code> and</li>
  <li>second column <code>t[:1]</code> is not <code>[2, 5]</code> and</li>
  <li>second column <code>t[:3]</code> is not <code>[3, 6]</code>
  ?
  How to conceptualise this?</li>
  </ul>