Raymond Hemmecke Jason Morton Anne Shiu Bernd Sturmfels Oliver Wienand Three Counter-Examples on Semi-Graphoids. 239-257 2008 17 Combinatorics, Probability & Computing 2 http://dx.doi.org/10.1017/S0963548307008838 db/journals/cpc/cpc17.html#HemmeckeMSSW08