Grouping with nested queries

Queries can be nested arbitrarily:

  WHERE <book> $p </> IN "bib.xml",
  
        <title> $t </>
        <publisher> <name> "Addison-Wesley" </> </> IN $p
  CONSTRUCT <result>
               <title> $t </>
               { WHERE  <author> $a </> IN $p
                 CONSTRUCT <author> $a </> 
               }
            </>

$p matches the contents of each book.
$t matches titles of Addison-Wesley books.
each title is included once in the reault.
in the nested query, $a matches all authors of the given title.

This will group authors of the same book:

<XML> <result> <title> An Introduction to Database Systems </title> <author> <lastname> Date </lastname> </author> </result> <result> <title> Foundation for Object/Relational Databases</title> <author> <lastname> Date </lastname> </author> <author> <lastname> Darwen </lastname> </author> </result> </XML>