Teodor Knapik Damian Niwinski Pawel Urzyczyn Igor Walukiewicz Unsafe Grammars and Panic Automata. 1450-1461 2005 conf/icalp/2005 ICALP http://dx.doi.org/10.1007/11523468_117 db/conf/icalp/icalp2005.html#KnapikNUW05