color(red).
color(green).
color(blue).
color(yellow).
 
adjacent(鶴見, 神奈川).
adjacent(神奈川, 西).
adjacent(西, 中).
adjacent(中, 南).
adjacent(南, 港南).
adjacent(港南, 磯子).
adjacent(磯子, 金沢).
adjacent(金沢, 栄).
adjacent(栄, 港南).
adjacent(中, 保土ケ谷).
adjacent(保土ケ谷, 西).
adjacent(保土ケ谷, 旭).
adjacent(旭, 瀬谷).
adjacent(瀬谷, 泉).
adjacent(泉, 戸塚).
adjacent(戸塚, 栄).
adjacent(都筑, 港北).
adjacent(都筑, 青葉).
adjacent(都筑, 緑).
adjacent(港北, 神奈川).
adjacent(緑, 青葉).
adjacent(港北, 鶴見).
adjacent(保土ケ谷, 緑).
 
different(X-Y, Z-W) :-
    (adjacent(X, Z); adjacent(Z, X)) -> Y \= W ; true.
 
coloring([
    鶴見-A, 神奈川-B, 西-C, 中-D, 南-E, 港南-F,
    磯子-G, 金沢-H, 栄-I, 保土ケ谷-J, 旭-K, 瀬谷-L,
    泉-M, 戸塚-N, 都筑-O, 港北-P, 緑-Q, 青葉-R
]) :-
    color(A), color(B), color(C), color(D),
    color(E), color(F), color(G), color(H),
    color(I), color(J), color(K), color(L),
    color(M), color(N), color(O), color(P),
    color(Q), color(R),
 
    Vars = [
        鶴見-A, 神奈川-B, 西-C, 中-D, 南-E, 港南-F,
        磯子-G, 金沢-H, 栄-I, 保土ケ谷-J, 旭-K, 瀬谷-L,
        泉-M, 戸塚-N, 都筑-O, 港北-P, 緑-Q, 青葉-R
    ],
 
    % 制約を先にまとめて課す
    forall(
        (member(X1-Y1, Vars), member(X2-Y2, Vars), X1 @< X2),
        different(X1-Y1, X2-Y2)
    ),
 
    % 出力
    write('4色で塗り分け可能'), nl,
    print_colors(Vars).
 
print_colors([]).
print_colors([区-色 | 残り]) :-
    write(区), write(' -> '), write(色), nl,
    print_colors(残り).
 
:- coloring(_).
 

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