Single-Type Approximations of Regular Tree Languages

XML Schema Definitions can be adequately abstracted by the single-type regular tree languages, which form a strict subclass of regular unranked tree languages. Sadly, in this respect, XSDs are not closed under the basic operations of union and set difference, complicating important task in schema integration and evolution. We investigate how these operations can be approximated within the framework of single-type regular tree languages. We consider both lower and upper approximations.