title: How to use `argparse` to set any 2 out of 3 arguments?
body: <p>There are three arguments: <code>A</code>, <code>B</code>, <code>C</code>. How can I use <code>argparse</code> to allow for any two of them to be set and return an error if all three are set?</p>
  
  <p>If I use the method <code>add_mutually_exclusive_group</code>, then I think I can only set one and only one of the three arguments. Is there a way to overcome this?</p>