/* properties and values */

  girl(red).
  girl(green).
  girl(blue).

/* constraints */

  a(red, green).
  c(red, X) :- X \= red.
  d(green, X) :- X \= green.
  e(blue, X) :- X \= blue.

/* solve predicate */

solve(Boy1,Girl1, Boy2,Girl2, Boy3,Girl3) :-

  /* freeze first property */

  Boy1 = red,
  Boy2 = green,
  Boy3 = blue,

  /* make values of other properties distinct */

  girl(Girl1),
  girl(Girl2),
  girl(Girl3),

  Girl1 \= Girl2, Girl1 \= Girl3,
  Girl2 \= Girl3,

  /* assert constraints */

  ( a(Boy1, Girl1);
    a(Boy2, Girl2);
    a(Boy3, Girl3) ),

  ( c(Boy1, Girl1);
    c(Boy2, Girl2);
    c(Boy3, Girl3) ),

  ( d(Boy1, Girl1);
    d(Boy2, Girl2);
    d(Boy3, Girl3) ),

  ( e(Boy1, Girl1);
    e(Boy2, Girl2);
    e(Boy3, Girl3) ).

/* go/0 predicate, prints all solutions */

go :-
  solve(Boy1,Girl1, Boy2,Girl2, Boy3,Girl3),

  /* print a solution */

  write(Boy1), write('\t'),
  write(Girl1), write('\n'),

  write(Boy2), write('\t'),
  write(Girl2), write('\n'),

  write(Boy3), write('\t'),
  write(Girl3), write('\n'),

  /* backtrack for all solutions */

  write('\n'), fail.