title: Python ArgumentParser nested arguments
body: <p>I want to create argument parser with following signature:</p>
  
  <p><code>./myapp [-a [-b BVAL] | -c]</code></p>
  
  <p>In other words, user could provide argument <code>-b BVAL</code> only in case if he provided argument <code>-a</code>. </p>
  
  <p>It's quite easy to create mutually exclusive group of <code>-a</code> and <code>-c</code>, but I can't figure out how to create relationship <code>allow -b only if -a provided</code></p>