title: Pandas: drop a level from a multi-level column index?
body: <p>If I've got a multi-level column index:</p>
  
  <pre><code>&gt;&gt;&gt; cols = pd.MultiIndex.from_tuples([("a", "b"), ("a", "c")])
  &gt;&gt;&gt; pd.DataFrame([[1,2], [3,4]], columns=cols)
  </code></pre>
  
  <pre>
      a
     ---+--
      b | c
  --+---+--
  0 | 1 | 2
  1 | 3 | 4
  </pre>
  
  <p>How can I drop the "a" level of that index, so I end up with:</p>
  
  <pre>
      b | c
  --+---+--
  0 | 1 | 2
  1 | 3 | 4
  </pre>