title: Find elements in a list that are not in the second list (in scala)
body: <p>Suppose I have two lists:</p>
  
  <pre><code>val a = List('a', 'b', 'c')
  val b = List('a', 'b', 'c', 'd')
  </code></pre>
  
  <p>I want to get the element which is not in the first list (in this case it's 'd'). I know I can do this with a loop, but is there any fancy functional way to do this quickly in one line?</p>
  
  <p>I've been looking at the Scala List API, but could only found union and intersection (which will give me List('a', 'b', 'c', 'd') and List('a', 'b', 'c') respectively)</p>