Javier Leach Albert Burkhard Monien Mario Rodríguez-Artalejo Automata, Languages and Programming, 18th International Colloquium, ICALP91, Madrid, Spain, July 8-12, 1991, Proceedings ICALP Lecture Notes in Computer Science 510 Springer 1991 3-540-54233-7 db/conf/icalp/icalp91.html