@incollection{Wand_1975,
    doi = {10.1007/3-540-07142-3_84},
    url = {https://doi.org/10.1007%2F3-540-07142-3_84},
    year = {1975},
    publisher = {Springer Berlin Heidelberg},
    pages = {209--213},
    author = {Mitchell Wand},
    title = {An algebraic formulation of the Chomsky hierarchy},
    booktitle = {Lecture Notes in Computer Science}
}

@incollection{Wand_1975, doi = {10.1007/3-540-07142-3_84}, url = {https://doi.org/10.1007%2F3-540-07142-3_84}, year = {1975}, publisher = {Springer Berlin Heidelberg}, pages = {209--213}, author = {Mitchell Wand}, title = {An algebraic formulation of the Chomsky hierarchy}, booktitle = {Lecture Notes in Computer Science} }