%% representing the graph

node(1).
node(2).
node(3).
node(4).
node(5).

edge(1,2).
edge(1,3).
edge(2,4).
edge(2,5).
edge(3,4).
edge(3,5).

is_color(red).
is_color(green).
is_color(yellow).

%% each node is assigned a color

color(X,red):- node(X), not color(X,green), not color(X, yellow).
color(X,green):- node(X), not color(X,red), not color(X, yellow).
color(X,yellow):- node(X), not color(X,green), not color(X, red).

%% constraint checking 

:- edge(X,Y), is_color(C), color(X,C), color(Y,C).