% Graph is specified as facts
vertex(a).
vertex(b).
vertex(c).
edge(a, b).
edge(b, c).
edge(c, a).

% Edges are undirected
edge(Y, X) :- edge(X, Y).

% Guess color for each vertex
red(V) :- vertex(V), not green(V), not blue(V).
green(V) :- vertex(V), not red(V), not blue(V).
blue(V) :- vertex(V), not red(V), not green(V).

% Check for invalid colorings
:- vertex(V1), vertex(V2), edge(V1, V2), red(V1), red(V2).
:- vertex(V1), vertex(V2), edge(V1, V2), green(V1), green(V2).
:- vertex(V1), vertex(V2), edge(V1, V2), blue(V1), blue(V2).