title: opposite of df.diff() in pandas
body: <p>I have searched the forums in search of a cleaner way to create a new column in a dataframe that is the sum of the row with the previous row- the opposite of the .diff() function which takes the difference.  </p>
  
  <p>this is how I'm currently solving the problem</p>
  
  <pre><code>df = pd.DataFrame ({'c':['dd','ee','ff', 'gg', 'hh'], 'd':[1,2,3,4,5]}
  df['e']= df['d'].shift(-1)
  df['f'] = df['d'] + df['e']
  </code></pre>
  
  <p>Your ideas are appreciated.</p>