Liang Zhao Xiangpeng Zhao Quan Long Zongyan Qiu A Type System for the Relational Calculus of Object Systems. 189-198 2006 conf/iceccs/2006 ICECCS http://doi.ieeecomputersociety.org/10.1109/ICECCS.2006.47 db/conf/iceccs/iceccs2006.html#ZhaoZLQ06