title: check if elements of string exist in another string
body: <p>I have two strings:</p>
  
  <pre><code>l1='a1 a2 b1 b2 c1 c2'
  l2='a1 b3 c1'
  </code></pre>
  
  <p>And I want to check if each element of string <code>l2</code> exists in <code>l1</code>, and then remove it from <code>l1</code>.</p>
  
  <p>Is it possible to do that without a <code>for</code> loop?</p>