title: Finding All Positions for Multiple Elements in a Vector
body: <p>Suppose I have the following vector:</p>
  
  <pre><code>x &lt;- c(8, 6, 9, 9, 7, 3, 2, 5, 5, 1, 6, 8, 5, 2, 9, 3, 5, 10, 8, 2)
  </code></pre>
  
  <p>How can I find which elements are either 8 or 9?</p>