Yingcang Ma Qiaoyan Li A Propositional Deductive System Of Universal Logic with Projection Operator. 996-998 2006 conf/isda/2006 ISDA (1) https://doi.org/10.1109/ISDA.2006.77 https://doi.ieeecomputersociety.org/10.1109/ISDA.2006.77 db/conf/isda/isda2006-1.html#MaL06