%query: conf(i). /* from Stefaan Decorte, Danny De Schreye, Henk Vandecasteele, 1997 */ conf(X) :- del2(X,Z), del(U,Y,Z), conf(Y). del2(X,Y) :- del(U,X,Z), del(V,Z,Y). del(X,[X|T],T). del(X,[H|T],[H|T1]) :- del(X,T,T1). s2l(s(X),[Y|Xs]):- s2l(X,Xs). s2l(0, []). goal(X) :- s2l(X,XS), conf(XS).