title: How do I use clojure.set/difference? Why won't it work on a PersistentSet?
body: <p>The following code:</p>
  
  <pre><code>(require '[clojure.set])
  (println (clojure.set/difference '("a" "b" "c" "d") '("c" "d" "e" "f")))
  </code></pre>
  
  <p>gives me the following error:</p>
  
  <pre><code>java.lang.ClassCastException: clojure.lang.PersistentList (repl-1:47)
  </code></pre>
  
  <p>I don't understand what I'm doing wrong.  Shouldn't this print out ("a" "b")?</p>