Joseph Gil John Howse Elena Tulchinsky Positive Semantics of Projections in Venn-Euler Diagrams. 197-227 2002 13 J. Vis. Lang. Comput. 2 http://www.idealibrary.com/links/doi/10.1006/jvlc.2000.0199 db/journals/vlc/vlc13.html#GilHT02