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Summary: Better Quasi-Ordered Transition Systems
Parosh Aziz Abdulla Aletta Nylen
Dept. of Information Technology, Uppsala University
P.O. Box 337, SE-751 05 Uppsala, Sweden
Email:fparosh, alettag@it.uu.se
Abstract
Many existing algorithms for model checking of in nite-state sys-
tems operate on constraints which are used to represent (potentially
in nite) sets of states. A general powerful technique which can be
employed for proving termination of these algorithms is that of well
quasi-orderings. Several methodologies have been proposed for deriva-
tion of new well quasi-ordered constraint systems. However, many of
these constraint systems su er from a \constraint explosion problem",
as the number of the generated constraints grows exponentially with
the size of the problem. In this paper, we demonstrate that a re ne-
ment of the theory of well quasi-orderings, called the theory of bet-
ter quasi-orderings, is more appropriate for symbolic model checking,
since it allows inventing constraint systems which are both well quasi-
ordered and compact. As a main application, we introduce existential
zones, a constraint system for veri cation of systems with unbound-
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