@incollection{Wagner_1975,
    doi = {10.1007/3-540-07389-2_231},
    url = {https://doi.org/10.1007%2F3-540-07389-2_231},
    year = {1975},
    publisher = {Springer Berlin Heidelberg},
    pages = {445--449},
    author = {Klaus Wagner},
    title = {A hierarchy of regular sequence sets},
    booktitle = {Mathematical Foundations of Computer Science 1975 4th Symposium, Mari{\'{a}}nsk{\'{e}} L{\'{a}}zn{\v{e}}, September 1{\textendash}5, 1975}
}

@incollection{Wagner_1975, doi = {10.1007/3-540-07389-2_231}, url = {https://doi.org/10.1007%2F3-540-07389-2_231}, year = {1975}, publisher = {Springer Berlin Heidelberg}, pages = {445--449}, author = {Klaus Wagner}, title = {A hierarchy of regular sequence sets}, booktitle = {Mathematical Foundations of Computer Science 1975 4th Symposium, Mari{\'{a}}nsk{\'{e}} L{\'{a}}zn{\v{e}}, September 1{\textendash}5, 1975} }