title: Remove all the elements that occur in one list from another
body: <p>Let's say I have two lists, <code>l1</code> and <code>l2</code>.  I want to perform <code>l1 - l2</code>, which returns all elements of <code>l1</code> not in <code>l2</code>.</p>
  
  <p>I can think of a naive loop approach to doing this, but that is going to be really inefficient.  What is a pythonic and efficient way of doing this?</p>
  
  <p>As an example, if I have <code>l1 = [1,2,6,8] and l2 = [2,3,5,8]</code>, <code>l1 - l2</code> should return <code>[1,6]</code></p>