This paper presents the result of research on heuristic methods for assessment of fault-trees (Boolean formulae) using Binary Decision Diagrams (BDDs for short). are the state-of-the-art com-pact representation of booleam formulae. The fault trees under consideration are given in the sum-of-proucts or poroducts-of-sums form. We propose a new heuristic htat is both robust and discriminating on these particular formulae. The s izes of the BDD obtained are much smaller than those obtained using standard heuristics. However from the theoretical point of view the size of the corresponding BDD remains exponentally large in the worst case. Nevertheless, in practice we manage to deal with complex formulae by means of a new rewriting method which we propose in this article.
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