We address the problem of verifying clique avoidance in the TTP protocol. TTP allows several stations embedded in a car to communicate. It has many mechanisms to ensure robustness to faults. In particular, it has an algorithm that allows a station to recognize itself as faulty and leave the communication. This algorithm must satisfy the crucial ‘non-clique’ property: it is impossible to have two or more disjoint groups of stations communicating exclusively with stations in their own group. In this paper, we propose an automatic verification method for an arbitrary number of stations $N$ and a given number of faults $k$. We give an abstraction that allows to model the algorithm by means of unbounded (parametric) counter automata. We have checked the non-clique property on this model in the case of one fault, using the ALV tool as well as the LASH tool.

Constraint Handling Rules (CHR) is a committed-choice rule-based language that was originally intended for writing constraint solvers. In this paper we show that it is also possible to write the classic union-find algorithm and variants in CHR. The programs neither compromise in declarativeness nor efficiency. We study the time complexity of our programs: they match the almost-linear complexity of the best known imperative implementations. This fact is illustrated with experimental results.