title: Pandas reverse of diff()
body: <p>I have calculated the differences between consecutive values in a series, but I cannot reverse / undifference them using <code>diffinv()</code>:   </p>
  
  <pre><code>ds_sqrt = np.sqrt(ds)
  ds_sqrt = pd.DataFrame(ds_sqrt)
  ds_diff = ds_sqrt.diff().values
  </code></pre>
  
  <p>How can I undifference this?</p>