Alain Finkel Bernard Willems Pierre Wolper A direct symbolic approach to model checking pushdown systems. 1997 9 Electr. Notes Theor. Comput. Sci. http://www.elsevier.com/gej-ng/31/29/23/34/23/show/Products/notes/index.htt#006 db/journals/entcs/entcs9.html#FinkelWW97