Henrik Reif Andersen

Verification of Hierarchical State/Event Systems Using Reusability and Compositionality

We investigate techniques for verifying hierarchical systems, i.e., finite state systems with a nesting capability. The straightforward way of analysing a hierarchical system is to first flatten it into an equivalent non-hierarchical system and then apply existing finite state system verification techniques. Though conceptually simple, flattening is severely punished by the hierarchical depth of a system. To alleviate this problem, we develop a technique that exploits the hierarchical structure to reuse earlier reachability checks of superstates to conclude reachability of substates. We combine the reusability technique with the successful compositional technique of [13] and investigate the combination experimentally on industrial systems and hierarchical systems generated according to our expectations to real systems. The experimental results are very encouraging: whereas a flattening approach degrades in performance with an increase in the hierarchical depth (even when applying the technique of [13]), the new approach proves not only insensitive to the hierarchical depth, but even leads to improved performance as the depth increases.

Verification of Hierarchical State/Event Systems using Reusability and Compositionality

We investigate techniques for verifying hierarchical systems, i.e., finite state systems with a nesting capability. The straightforward way of analysing a hierarchical system is to first flatten it into an equivalent non-hierarchical system and then apply existing finite state system verification techniques. Though conceptually simple, flattening is severely punished by the hierarchical depth of a system. To alleviate this problem, we develop a technique that exploits the hierarchical structure to reuse earlier reachability checks of superstates to conclude reachability of substates. We combine the reusability technique with the successful compositional technique of J. Lind-Nielsen, H.R. Andersen, G. Behrmann, H. Hulgaard, K. Kristoffersen, and K.G. Larsen, 1998. In: Tools and Algorithms for the Construction and Analysis of Systems, Vol. 1384 of Lecture Notes in Computer Science, pp. 201–216, and investigate the combination experimentally on industrial systems and hierarchical systems generated according to our expectations to real systems. The experimental results are very encouraging: whereas a flattening approach degrades in performance with an increase in the hierarchical depth (even when applying the technique of J. Lind-Nielsen et al. (1998)), the new approach proves not only insensitive to the hierarchical depth, but even leads to improved performance as the depth increases.

Symbolic model checking of timed guarded commands using difference decision diagrams

Abstract We describe a novel methodology for analyzing timed systems symbolically. Given a formula representing a set of states, we describe how to determine a new formula that represents the set of states reachable by taking a discrete transition or by advancing time. The symbolic representations are given as formulae expressed in a simple first-order logic over difference constraints of the form x − y ⩽ d which can be combined with Boolean operators and existentially quantified. We also show how to symbolically determine the set of states that can reach a given set of states (i.e., a backward step), thus making it possible to verify timed ctl -formulae symbolically. The main contribution is a way of advancing time symbolically essentially by quantifying out a special variable z which is used to represent the current zero point in time. We also describe a data structure called ddd s for representing difference constraint formulae, and we demonstrate the efficiency of the symbolic technique by analyzing two scheduling protocols using a ddd -based model checker.

Using configuration technology as the core of a legal decision support system

This paper is concerned with the development of a logical model of a given legal act and the realization of a web-based decision support system on the basis of this model. The system is implemented using a configuration engine, which provides full propositional reasoning in polynomial time (after an off-line compilation step), and a good web-coupling infrastructure. To the best of our knowledge it is the first decision support system that is based on such an engine. Further issues that are addressed are the main problems that were encountered during the modeling of the act and the techniques that have been used to overcome them, as well as a number of user interface issues that are of crucial importance if such a system is to be accepted by its users.