Anders Møller Michael I. Schwartzbach The Design Space of Type Checkers for XML Transformation Languages. 17-36 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3363&spage=17 2005 conf/icdt/2005 ICDT db/conf/icdt/icdt2005.html#MollerS05