title: selecting from multi-index pandas
body: <p>I have a multi-index data frame with columns  'A' and 'B'. </p>
  
  <p>Is there is a way to select rows by filtering on one column of the multi-index without resetting the index to a single column index? </p>
  
  <p>For Example.</p>
  
  <pre><code># has multi-index (A,B)
  df
  #can I do this? I know this doesn't work because the index is multi-index so I need to     specify a tuple
  
  df.ix[df.A ==1]
  </code></pre>