title: How Can I combine two columns is one dataframe?
body: <p>I have a dataset like this.</p>
  <pre><code>A  B  C  A2
  1  2  3   4
  5  6  7   8
  </code></pre>
  <p>and I want to combine A and A2.</p>
  <pre><code>A  B  C
  1  2  3
  5  6  7
  4 
  8 
  </code></pre>
  <p>how can I combine two columns?
  Hope for help. Thank you.</p>