Xiao He Zhiyi Ma Weizhong Shao Ge Li A metamodel for the notation of graphical modeling languages. 219-224 2007 conf/compsac/2007 COMPSAC (1) http://doi.ieeecomputersociety.org/10.1109/COMPSAC.2007.27 db/conf/compsac/compsac2007-1.html#HeMSL07