Semistructured data may be irregular and incomplete and does not
  necessarily conform to a fixed schema.  As with structured data, it
  is often desirable to maintain a history of changes to data, and to
  query over both the data and the changes. Representing and querying
  changes in semistructured data is more difficult than in structured
  data due to the irregularity and lack of schema.  We present a model
  for representing changes in semistructured data and a language for
  querying over these changes.  An important feature of our approach
  is that we represent and query changes directly as annotations on
  the affected data, instead of indirectly as the difference between
  database states.  We describe the implementation of our model and
  query language.  We also describe the design and implementation of a
  query subscription service that permits users to subscribe
  to changes in semistructured information sources.