%% search.pl

start([]).
goal(S) :- length(S,8).

next_state(S,[C|S]) :- member(C,[1,2,3,4,5,6,7,8]),
                       not member(C,S).

safe_state([C|S]) :- length(S,L),
                     Sum is C+L+1, Diff is C-L-1,
                     safe_state(S,Sum,Diff).


safe_state([],_,_) :- !.
safe_state([F|R],Sm,Df) :- length(R,L),
                           X is F+L+1,
                           X \= Sm,
                           Y is F-L-1,
                           Y \= Df,
                           safe_state(R,Sm,Df).