title: Detect specific value change in Pandas DataSeries
body: <p>I want to detect a specific change of the value of a DataSeries in Pandas.
  Given I got a DataSeries of the following Format:</p>
  <pre><code>ds = {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 1, 'f': 5, 'g': 2}
  </code></pre>
  <p>With DataSeries built in diff(1) function I am able to detect a value change and how big it was. Is it possible to only get the occurances where the value changes from 4 to 1?</p>