title: pandas dataframe select columns in multiindex
body: <p>I have the following pd.DataFrame:</p>
  
  <pre><code>Name    0                       1                      ...
  Col     A           B           A            B         ...
  0       0.409511    -0.537108   -0.355529    0.212134  ...
  1       -0.332276   -1.087013    0.083684    0.529002  ...
  2       1.138159    -0.327212    0.570834    2.337718  ...
  </code></pre>
  
  <p>It has MultiIndex columns with <code>names=['Name', 'Col']</code> and hierarchical levels. The <code>Name</code> label goes from 0 to n, and for each label, there are two <code>A</code> and <code>B</code> columns. </p>
  
  <p>I would like to subselect all the <code>A</code> (or <code>B</code>) columns of this DataFrame. </p>