title: Extend Clojure protocol to a primitive array
body: <p>I'd like to extend a Clojure protocol to deal with Java primitive arrays.</p>
  
  <pre><code>(defprotocol PVectorisable
    (to-vector [a]))
  
  (extend-protocol PVectorisable
    ??????
      (to-vector [coll]
        (Vectorz/create ^doubles coll))
    java.util.List
      ... other implementations......)
  </code></pre>
  
  <p>Is this possible, and if so what needs to go in the extend-protocol definition above (in place of "??????")?</p>