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

coloring(A, B, C) :-
  color(A),
  color(B),
  neq(A, B),
  color(C),
  neq(B, C),
  neq(A, C).

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