title: How can I define multiple requirement files?
body: <p>How can I do define multiple requirements files in my <code>requirements.txt</code> file.</p>
  
  <pre><code>-r base.txt
  -r test.txt
  </code></pre>
  
  <p>The current behavior is that <code>pip</code> only installs packages from <code>test.txt</code>. I'd expect pip to install packages found in both <code>base.txt</code> and <code>test.txt</code>. I could have sworn I've seen someone do this in the past, but I can't find any examples.</p>