title: Selecting first different value in panda series
body: <p>I have a Panda Series such as</p>
  <pre><code>0  A
  1  A       
  2  B       
  3  B       
  4  A
  5  B
  6  B
  7  B
  8  A   
  </code></pre>
  <p>I need to select the first different value every time, so the result should look like</p>
  <pre><code>0  A   
  2  B       
  4  A
  5  B
  8  A
  </code></pre>
  <p>I can do this with a simple loop and a tracking variable but I need to apply this to massive dataframes/series. Is there a vectorized/better way of doing this?</p>