title: Getting all rows with NaN value
body: <p>I have a table with a column that has some NaN values in it:</p>
  
  <pre><code>A   B   C   D
  2   3   2   Nan
  3   4   5   5
  2   3   1   Nan
  </code></pre>
  
  <p>I'd like to get all rows where D = NaN. How can I do this?</p>