title: Remove elements of one list from another, while keeping duplicates
body: <p>Basically what I want is this:</p>
  
  <pre><code>&gt;&gt;&gt; a = ["a","a","b","c","c","c","d","e","f"]
  &gt;&gt;&gt; b = ["a","b","c","d","e","f"]
  &gt;&gt;&gt; #Do something, something like a - b
  &gt;&gt;&gt; result = ["a","c","c"]
  </code></pre>
  
  <p>Reason I want to do this, I am joining a bunch of lists of preferences and want to find which one is common among a lot of lists. The more times they occur in list a (because more lists have that element) the more weight I put towards that</p>