  The work beginning with  Lifschitz 1987  has contributed to a sustained line of research in the causal approach—not only by Lifschitz and students of his such as Enrico Giunchiglia and Hudson Turner, but by researchers at other sites. For work in this area, and further references, see  Thielscher 1989,  Gustaffson &amp; Doherty 1996,  Baral 1995,  Nakashima et al. 1997,  Lifschitz 1997,  Giunchiglia &amp; Lifschitz 1998,  Lin 1995,  Haugh 1987,  Lifschitz 1998,  Turner 1999,  McCain &amp; Turner 1995,  Elkan 1991,  McCain &amp; Turner 1997,  Thielscher 1996,  and  Gelfond &amp; Lifschitz 1998.