title: Operator expected error in xpath queries in SwiProlog
body: <p>I am attempting to use the xpath predicate to pull out a portion of a parsed XML file, but keep getting Syntax error: Operator expected on the special characters (e.g. "/" and "@") the xpath query.  WHen I surround the query with single quotes it fails.</p>
  
  <pre><code>?- load_xml_file( 'books.xml', DOM ).
  DOM = [element(book, [genre=romance], ['\n   Gone With The Wind\n']), element(book, [genre=thriller], ['\n   The Shining\n'])].
  
  ?- load_xml_file( 'books.xml', DOM ), xpath(DOM, //book(@genre=thriller), Book).
  ERROR: Syntax error: Operator expected
  ERROR: load_xml_file( 'books.xml', DOM ), xpath(DOM, //
  ERROR: ** here **
  ERROR: book(@genre=thriller), Book) . 
  
  ?- load_xml_file( 'books.xml', DOM ), xpath(DOM, '//book(@genre=thriller)', Book).
  false.
  </code></pre>
  
  <p>The contents of books.xml:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;book genre="romance"&gt;
     Gone With The Wind
  &lt;/book&gt;
  &lt;book genre="thriller"&gt;
     The Shining
  &lt;/book&gt;
  </code></pre>