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#external color(1).
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#external edge(2).
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#external vertex(1).
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#show color/2.
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{color(V, C)} :- vertex(V), color(C).
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covered(V) :- color(V, _).
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:- vertex(V), not covered(V).
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:- color(V1, C), color(V2, C), edge(V1, V2).
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:- color(V, C1), color(V, C2), C1 != C2.
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