Andrew Fish, Gem Stapleton. Defining Euler Diagrams: Simple or What?. In Dave Barker-Plummer, Richard Cox, Nik Swoboda, editors, Diagrammatic Representation and Inference, 4th International Conference, Diagrams 2006, Stanford, CA, USA, June 28-30, 2006, Proceedings. Volume 4045 of Lecture Notes in Computer Science, pages 109-111, Springer, 2006. [doi]