title: Loop macro: Parentheses in the using subclause
body: <p><a href="http://www.gigamonkeys.com/book/loop-for-black-belts.html" rel="nofollow noreferrer">Chapter 22</a> of Practical Common Lisp explains (among other things) how to use both the keys and the values when iterating through a hash table with the <code>loop</code> macro:</p>
  
  <pre><code>(loop for k being the hash-keys in h using (hash-value v) ...)
  </code></pre>
  
  <p>The explanation is concluded with the following footnote:</p>
  
  <blockquote>
    <p>Don't ask me why <code>LOOP</code>'s authors chickened out on the no-parentheses style for the <code>using</code> subclause.</p>
  </blockquote>
  
  <p>That's perfectly fine for the book and I don't blame Peter Seibel for not knowing it. Still, I am somewhat curious about the reasons for this design decision of. Why isn't the syntax something like</p>
  
  <pre><code>(loop for k being the hash-keys in h using hash-value v ...)
  </code></pre>
  
  <p>Is it easier to parse? Or is there some deeper reason behind it?</p>