@article{Hibbard:Ullian:UCFL:1966,
    Address = {New York, NY, USA},
    Author = {Hibbard, Thomas N. and Ullian, Joseph},
    File = {p588-hibbard (0) - a - a - i.pdf},
    ISSN = {0004-5411},
    Journal = {J. ACM},
    Month = {October},
    Number = {4},
    Pages = {588--593},
    Publisher = {ACM},
    Title = {The Independence of Inherent Ambiguity From Complementedness Among Context-Free Languages},
    URL = {http://doi.acm.org/10.1145/321356.321366},
    Volume = {13},
    Year = {1966},
    acmid = {321366},
    bdsk-url-1 = {http://doi.acm.org/10.1145/321356.321366},
    bdsk-url-2 = {https://doi.org/10.1145/321356.321366},
    date-added = {2018-06-08 12:28:38 +0000},
    date-modified = {2018-10-04 12:41:39 +0000},
    issue_date = {Oct. 1966},
    numpages = {6},
    doi = {10.1145/321356.321366}
}

@article{Hibbard:Ullian:UCFL:1966, Address = {New York, NY, USA}, Author = {Hibbard, Thomas N. and Ullian, Joseph}, File = {p588-hibbard (0) - a - a - i.pdf}, ISSN = {0004-5411}, Journal = {J. ACM}, Month = {October}, Number = {4}, Pages = {588--593}, Publisher = {ACM}, Title = {The Independence of Inherent Ambiguity From Complementedness Among Context-Free Languages}, URL = {http://doi.acm.org/10.1145/321356.321366}, Volume = {13}, Year = {1966}, acmid = {321366}, bdsk-url-1 = {http://doi.acm.org/10.1145/321356.321366}, bdsk-url-2 = {https://doi.org/10.1145/321356.321366}, date-added = {2018-06-08 12:28:38 +0000}, date-modified = {2018-10-04 12:41:39 +0000}, issue_date = {Oct. 1966}, numpages = {6}, doi = {10.1145/321356.321366} }