@incollection{Valk_1978,
    doi = {10.1007/3-540-08860-1_35},
    url = {https://doi.org/10.1007%2F3-540-08860-1_35},
    year = {1978},
    publisher = {Springer Berlin Heidelberg},
    pages = {464--476},
    author = {Rüdiger Valk},
    title = {Self-modifying nets, a natural extension of Petri nets},
    booktitle = {Automata, Languages and Programming}
}

@incollection{Valk_1978, doi = {10.1007/3-540-08860-1_35}, url = {https://doi.org/10.1007%2F3-540-08860-1_35}, year = {1978}, publisher = {Springer Berlin Heidelberg}, pages = {464--476}, author = {Rüdiger Valk}, title = {Self-modifying nets, a natural extension of Petri nets}, booktitle = {Automata, Languages and Programming} }