db/journals/jacm/AptE82.html Krzysztof R. Apt Maarten H. van Emden Contributions to the Theory of Logic Programming. 841-862 1982 29 J. ACM 3 http://doi.acm.org/10.1145/322326.322339 db/journals/jacm/jacm29.html#AptE82 JACM/29/P0841.pdf