Ahmed Bouajjani Rachid Echahed Riadh Robbana Verification of Context-Free Timed Systems Using Linear Hybrid Observers. 118-131 1994 conf/cav/1994 CAV db/conf/cav/cav94.html#BouajjaniER94 http://dx.doi.org/10.1007/3-540-58179-0_48