title: Why does my list inclusion negation not work?
body: <p>As part of a (Python 3.3) program I am writing, I am comparing two lists; I want to find every member of the second list that does not occur in the first list so that I can build a database command.  The "in" membership test works as I expect, but its negation does not.  </p>
  
  <p>For instance:</p>
  
  <pre><code>l1 = ['MNE11B', 'DLC05A', 'MKH08A', 'PERGDM', 'DLW12B', 'FSW08A', 'PNC12A', 'KHH04A', 'BWD12A', 'YDB12A']
  l2 = ['MKH08A', 'FSW08A', 'BWD12A']
  
  print("Present")
  for x in l2:
      if x in l1:
          print(x)
  
  print("Not present")
  for x in l2:
      if x not in l1:
          print(x)
  </code></pre>
  
  <p>The first "for" loop prints out the three members in l2 that also exist in l1.  I would expect the second loop to print out the members in l2 that are not in l1; it instead prints nothing at all.  Why is this?  I have tried various syntactical tricks with parentheses and such, but this is being stubborn.</p>