title: Common Lisp: convert between lists and arrays
body: <p>How do we convert elegantly between arbitrarily nested lists and arrays?</p>
  
  <p>e.g.</p>
  
  <pre><code>((1 2 3) (4 5 6))
  </code></pre>
  
  <p>becomes</p>
  
  <pre><code>#2A((1 2 3) (4 5 6))
  </code></pre>
  
  <p>and vice versa</p>