title: How to select multi range of rows in pandas dataframe
body: <p>Given an example of pandas dataframe with index from <code>0</code> to <code>30</code>. I would like to select the rows within several ranges of index, <code>[0:5]</code>, <code>[10:15]</code> and <code>[20:25]</code>.</p>
  
  <p>How to do that?</p>