Grouping in XML.
Stelios Paparizos, Shurug Al-Khalifa, H. V. Jagadish, Laks V.S. Lakshmanan,
Andrew Nierman, Divesh Srivastava and Yuqing Wu.

XML permits repeated and missing sub-elements, and missing attributes.  
We discuss the consequent implications on grouping, both with respect 
to specification and with respect to implementation.  The techniques 
described here have been implemented in the TIMBER native XML database 
system being developed at the University of Michigan.