title: How to refer property-list attribute with a string
body: <p>Consider the following case</p>
  
  <pre><code>(setf mat (list :f1 1 :f2 2))
  </code></pre>
  
  <p><code>(getf mat :f1)</code>  outputs  <code>1</code> as expected.</p>
  
  <p>I have a variable <code>(setf str "f1")</code>  or <code>(setf str 'f1)</code> , whichever works.
  And I want to be able to do something like</p>
  
  <pre><code>(getf mat :str)
  </code></pre>
  
  <p>How can I do this?</p>