safe(Boardsize, Configuration) :- 
                    buildupto(Boardsize, RevConfig, 0, []),
                    reverse(RevConfig, Configuration).

buildupto(Boardsize, Configuration, Boardsize, Configuration).
buildupto(Boardsize, Configuration, Currentcol, PartialConfig) :-
   Newcol is Currentcol+1, ibt(1, Boardsize, Newrow),
   noclash([Newrow, Newcol], PartialConfig),
   buildupto(Boardsize, Configuration, Newcol, 
                              [[Newrow, Newcol] | PartialConfig]).

noclash([I,J], []).
noclash([I,J], [[K,L] | X]) :- I\==K, J\==L,
                               D1 is I-J, D2 is K-L, D1\==D2,
                               S1 is I+J, S2 is K+L, S1\==S2,
                               noclash([I,J], X).


ibt(L, H, I) :- number(I), !, L =< I, I =< H.
ibt(L, H, I) :- I is L.
ibt(L, H, I) :- L1 is L+1, L1 =< H, ibt(L1, H, I).