Support for constraints on elements. Referential integrity. "A language for
specifying and enforcing constraints between arbitrary document elements."
"constraints specification and triggers." --> XML-schema
Inference or Semantic Mediation: Several papers worry about the problem of
semantic mismatch between the query and the data. E.g. extracting price
information where the price is expressed in different currencies. Finding
"related" or "similar" data. Problem of inference mainly from a RDF perspective.