David Maier Jin Li Peter A. Tucker Kristin Tufte Vassilis Papadimos Semantics of Data Streams and Operators. 37-52 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3363&spage=37 2005 conf/icdt/2005 ICDT db/conf/icdt/icdt2005.html#MaierLTTP05