@article{Walters_1989,
    doi = {10.1016/0022-4049(89)90151-5},
    url = {https://doi.org/10.1016%2F0022-4049%2889%2990151-5},
    year = {1989},
    month = {dec},
    publisher = {Elsevier {BV}},
    volume = {62},
    number = {2},
    pages = {199--203},
    author = {R.F.C. Walters},
    title = {A note on context-free languages},
    journal = {Journal of Pure and Applied Algebra}
}

@article{Walters_1989, doi = {10.1016/0022-4049(89)90151-5}, url = {https://doi.org/10.1016%2F0022-4049%2889%2990151-5}, year = {1989}, month = {dec}, publisher = {Elsevier {BV}}, volume = {62}, number = {2}, pages = {199--203}, author = {R.F.C. Walters}, title = {A note on context-free languages}, journal = {Journal of Pure and Applied Algebra} }