title: difference between two lists that include duplicates
body: <p>I have a problem with two lists which contain duplicates</p>
  
  <pre><code>a = [1,1,2,3,4,4]
  b = [1,2,3,4]
  </code></pre>
  
  <p>I would like to be able to extract the differences between the two lists ie. </p>
  
  <pre><code>c = [1,4]
  but if I do c = a-b I get c =[]
  </code></pre>
  
  <p>It should be trivial but I can't find out  :(
  I tried also to parse the biggest list and remove items from it when I find them in the smallest list but I can't update lists on the fly, it does not work either</p>
  
  <p>has anyone got an idea ?
  thanks</p>