title: How to simply add a column level to a pandas dataframe
body: <p>let say I have a dataframe that looks like this:</p>
  
  <pre><code>df = pd.DataFrame(index=list('abcde'), data={'A': range(5), 'B': range(5)})
   df
  Out[92]: 
     A  B
  a  0  0
  b  1  1
  c  2  2
  d  3  3
  e  4  4
  </code></pre>
  
  <p>Asumming that this dataframe already exist, how can I simply add a level 'C' to the column index so I get this:</p>
  
  <pre><code> df
  Out[92]: 
     A  B
     C  C
  a  0  0
  b  1  1
  c  2  2
  d  3  3
  e  4  4
  </code></pre>
  
  <p>I saw SO anwser like this <a href="https://stackoverflow.com/questions/15989281/python-pandas-how-to-combine-two-dataframes-into-one-with-hierarchical-column-i">python/pandas: how to combine two dataframes into one with hierarchical column index?</a> but this concat different dataframe instead of adding a column level to an already existing dataframe.</p>
  
  <p>-</p>