@article{Maurer_1969,
    doi = {10.1145/321510.321517},
    url = {https://doi.org/10.1145%2F321510.321517},
    year = {1969},
    month = {apr},
    publisher = {Association for Computing Machinery ({ACM})},
    volume = {16},
    number = {2},
    pages = {256--260},
    author = {Herman A. Maurer},
    title = {A Direct Proof of the Inherent Ambiguity of a Simple Context-Free Language},
    journal = {Journal of the {ACM}}
}

@article{Maurer_1969, doi = {10.1145/321510.321517}, url = {https://doi.org/10.1145%2F321510.321517}, year = {1969}, month = {apr}, publisher = {Association for Computing Machinery ({ACM})}, volume = {16}, number = {2}, pages = {256--260}, author = {Herman A. Maurer}, title = {A Direct Proof of the Inherent Ambiguity of a Simple Context-Free Language}, journal = {Journal of the {ACM}} }