title: Why can I not use a list as a dictionary key?
body: <p>Why can I not use a list as a dictionary key? </p>
  
  <p>hlst is a list.
  memo is a dict.</p>
  
  <pre><code>if not hlst in memo:
      # do something
  else:
      configurations = memo[hlst]
  </code></pre>
  
  <p>When I try it python tells me that hlist is unhashable. </p>