Angelo Perkusich Jorge C. A. de Figueiredo G-Nets: A petri net based approach for logical and timing analysis of complex software systems. 39-59 1997 39 Journal of Systems and Software 1 http://dx.doi.org/10.1016/S0164-1212(96)00162-8 db/journals/jss/jss39.html#PerkusichF97