Allen Van Gelder Negation as Failure using Tight Derivations for General Logic Programs. 109-133 1989 6 J. Log. Program. 1&2 db/journals/jlp/jlp6.html#Gelder89 http://dx.doi.org/10.1016/0743-1066(89)90032-0 preliminary version: SLP 1986: 127-138