Some design methods based on Petri nets modify the original specification by behaviour-preserving insertion of new transitions. If the unfolding prefix is used to analyse the net, it has to be re-unfolded after each modification, which is detrimental for the overall performance. The approach presented in this paper applies the transformations directly to the unfolding prefix, thus avoiding re-unfolding. This also helps in visualisation, since the application of the transformation directly to

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... prefix changes it in a way that was 'intuitively expected' by the user, while re-unfolding can dramatically change the shape of the prefix. Moreover, rigourous validity checks for several kinds of transition insertions are developed. These checks are performed on the original unfolding prefix, so one never has to backtrack due to the choice of a transformation which does not preserve the behaviour. Abstract Some design methods based on Petri nets modify the original specification by behaviour-preserving insertion of new transitions. If the unfolding prefix is used to analyse the net, it has to be re-unfolded after each modification, which is detrimental for the overall performance. The approach presented in this paper applies the transformations directly to the unfolding prefix, thus avoiding reunfolding. This also helps in visualisation, since the application of the transformation directly to the prefix changes it in a way that was 'intuitively expected' by the user, while re-unfolding can dramatically change the shape of the prefix. Moreover, rigourous validity checks for several kinds of transition insertions are developed. These checks are performed on the original unfolding prefix, so one never has to backtrack due to the choice of a transformation which does not preserve the behaviour. Abstract. Some design methods based on Petri nets modify the original specification by behaviour-preserving insertion of new transitions. If the unfolding prefix is used to analyse the net, it has to be re-unfolded after each modification, which is detrimental for the overall performance. The approach presented in this paper applies the transformations directly to the unfolding prefix, thus avoiding re-unfolding. This also helps in visualisation, since the application of the transformation directly to the prefix changes it in a way that was 'intuitively expected' by the user, while re-unfolding can dramatically change the shape of the prefix. Moreover, rigourous validity checks for several kinds of transition insertions are developed. These checks are performed on the original unfolding prefix, so one never has to backtrack due to the choice of a transformation which does not preserve the behaviour.