Four Color test

four_color.pl

:- table four_color/2.

% four color
four_color(Place, Color) :-
    neighbours(Place, Neighbours),
    member(Color, [red, green, blue, yellow]),
    \+ four_color(Place, _),
    \+ (member(Neighbour, Neighbours), four_color(Neighbour, Color)).

% map of European Union
neighbours(austria, [czech_republic, germany, hungary, italy, slovenia, slovakia]).
neighbours(belgium, [france, netherlands, luxemburg, germany, united_kingdom]).
neighbours(bulgaria, [romania, greece]).
neighbours(croatia, [slovenia, hungary]).
neighbours(cyprus, [greece]).
neighbours(czech_republic, [germany, poland, slovakia, austria]).
neighbours(denmark, [germany, sweden]).
neighbours(estonia, [finland, latvia, lithuania]).
neighbours(finland, [estonia, sweden]).
neighbours(france, [spain, belgium, luxemburg, germany, italy, united_kingdom]).
neighbours(germany, [netherlands, belgium, luxemburg, denmark, france, austria, poland, czech_republic]).
neighbours(greece, [bulgaria, cyprus]).
neighbours(hungary, [austria, slovakia, romania, croatia, slovenia]).
neighbours(ireland, [united_kingdom]).
neighbours(italy, [france, austria, slovenia]).
neighbours(latvia, [estonia, lithuania]).
neighbours(lithuania, [estonia, latvia, poland]).
neighbours(luxemburg, [belgium, france, germany]).
neighbours(malta, []).
neighbours(netherlands, [belgium, germany, united_kingdom]).
neighbours(poland, [germany, czech_republic, slovakia, lithuania]).
neighbours(portugal, [spain]).
neighbours(romania, [hungary, bulgaria]).
neighbours(slovakia, [czech_republic, poland, hungary, austria]).
neighbours(slovenia, [austria, italy, hungary, croatia]).
neighbours(spain, [france, portugal]).
neighbours(sweden, [finland, denmark]).
neighbours(united_kingdom, [ireland, netherlands, belgium, france]).

% test run
run :-
    (   four_color(Country, Color),
        portray_clause(four_color(Country, Color)),
        fail
    ;   halt
    ).