@article{Engelfriet_1977,
    doi = {10.1016/s0022-0000(77)80034-2},
    url = {https://doi.org/10.1016%2Fs0022-0000%2877%2980034-2},
    year = {1977},
    month = {dec},
    publisher = {Elsevier {BV}},
    volume = {15},
    number = {3},
    pages = {328--353},
    author = {Joost Engelfriet and Erik Meineche Schmidt},
    title = {{IO} and {OI}. I},
    journal = {Journal of Computer and System Sciences}
}

@article{Engelfriet_1977, doi = {10.1016/s0022-0000(77)80034-2}, url = {https://doi.org/10.1016%2Fs0022-0000%2877%2980034-2}, year = {1977}, month = {dec}, publisher = {Elsevier {BV}}, volume = {15}, number = {3}, pages = {328--353}, author = {Joost Engelfriet and Erik Meineche Schmidt}, title = {{IO} and {OI}. I}, journal = {Journal of Computer and System Sciences} }