title: how to keep elements of a list based on another list
body: <p>I have two lists looking like:</p>
  
  <pre><code>list1 = ['a','a','b','b','b','c','d','e','e','g','g']
  
  list2 = ['a','c','z','y']
  </code></pre>
  
  <p>What I want to do is to keep all those elements of list1 that are also in list2.
  the outcome should be:</p>
  
  <pre><code>outcome= ['a','a','c']
  </code></pre>