% From The Art of EyeProlog, Chapter 10.
color(red).
color(green).
color(blue).

coloring(A, B, C) :-
  color(A),
  color(B),
  (A \= B),
  color(C),
  (B \= C),
  (A \= C).

answer(colors(A, B, C)) :- coloring(A, B, C).