@inproceedings{mey-1969-preservation,
    title = "On the Preservation of Context-Free Languages in a Level-Based System",
    author = "Mey, Jacob",
    booktitle = "{I}nternational {C}onference on {C}omputational {L}inguistics {COLING} 1969: Preprint No. 20",
    month = sep,
    year = "1969",
    address = {S{\r{a}}nga S{\"a}by, Sweden},
    url = "https://preview.aclanthology.org/fix-sig-urls/C69-2001/"
}