title: How do I combine two dataframes?
body: <p>I have a initial dataframe <code>D</code>. I extract two data frames from it like this:</p>
  <pre><code>A = D[D.label == k]
  B = D[D.label != k]
  </code></pre>
  <p>I want to combine <code>A</code> and <code>B</code> into one DataFrame. The order of the data is not important. However, when we sample <code>A</code> and <code>B</code> from <code>D</code>, they retain their indexes from <code>D</code>.</p>