Krzysztof R. Apt Alessandro Pellegrini On the Occur-Check-Free Prolog Programs. 687-726 1994 16 ACM Trans. Program. Lang. Syst. 3 http://doi.acm.org/10.1145/177492.177673 db/journals/toplas/toplas16.html#AptP94 preliminary version: PLILP 1992: 69-86