@inproceedings{1046813,
    title = "Unknown but not undefined?",
    author = "Heule, Dirk van and Hoogewijs, Albert",
    year = "1997",
    pages = "175-179",
    publisher = "Πανεπιστήμιο Κύπρου",
    booktitle = "Α' Πανελλήνιο Συμπόσιο Λογικής: πρακτικά"
}