title: How to prevent docopt from swallowing an option?
body: <p>I'm trying to create a command-line interface using docopt. Here is a simplified version of my file:</p>
  
  <pre><code>#!/usr/bin/env python
  """
  Test program.
  
  Usage:
    test.py [options]
  
  Options:
    -a &lt;input&gt;    
    -b
    -c
    -d
  """
  import docopt
  
  print docopt.docopt(__doc__)
  </code></pre>
  
  <p>I essentially want to be able to specify any of the options, in any order. However, if I forget to specify the argument for the <code>-a</code> flag, then I get an output like this:</p>
  
  <pre><code>$ python test.py -a -b -c
  {"-a": "-b",
   "-b": False,
   "-c": True,
   "-d": False}
  </code></pre>
  
  <p>Docopt is treating the <code>-b</code> flag as the argument for the <code>-a</code> flag, instead of rejecting the input as invalid. Is there some easy way to detect this, or make docopt refuse to accept this sort of malformed input?</p>