title: Remove duplicates of pairs in a list
body: <p>I am familiar with the <code>nub</code> function on lists containing numbers, characters, or strings, but 
  can someone explain to me how I can use the <code>nub</code> function from <code>Data.List</code> on a list of pairs?</p>
  
  <p>Example: </p>
  
  <pre><code>[('a', 3),( 'b', 2),('a', 1),('b', 4)]
  </code></pre>
  
  <p>to</p>
  
  <pre><code>[('a', 3),('b', 2)]
  </code></pre>
  
  <p>As you can see, I want to remove all pairs where the key from the pair (key, value) is already in the list.</p>