XML : MarkLogic: XQuery to Get Distinct Names from XML Document?

I am using the following file:

  <bookstore>  <book category="COOKING">  <title lang="en">Everyday Italian</title>  <author>Giada De Laurentiis</author>  <year>2005</year>  <price>30.00</price>  </book>  <book category="CHILDREN">  <title lang="en">Harry Potter</title>  <author>J K. Rowling</author>  <year>2005</year>  <price>29.99</price>  </book>  <book category="WEB">  <title lang="en">XQuery Kick Start</title>  <author>James McGovern</author>  <author>Per Bothner</author>  <author>Kurt Cagle</author>  <author>James Linn</author>  <author>Vaidyanathan Nagarajan</author>  <year>2003</year>  <price>49.99</price>  </book>  <book category="WEB">  <title lang="en">Learning XML</title>  <author>Erik T. Ray</author>  <year>2003</year>  <price>39.95</price>  </book>  </bookstore>    

I am using the following query to get the names from the XML document

  for $x at $i in doc("bookstore.xml")/bookstore/book/*  return fn:distinct-values(name($x))    

I am getting following results:

  title  author  year  price  title  author  year  price  title  author  author  author  author  author  year  price  title  author  year  price    

Instead I just want the distinct one as shown below.

  title  author  year  price    

I believe I messed up the for loop. Can someone help me with a fix for this? I tried using distinct-values(). No luck.