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:- object(colours,
implements(comparingp)).
:- info([
author is 'Paulo Moura',
version is 1.0,
date is 2000/4/22,
comment is 'Implements comparison between visible colors.']).
Colour1 < Colour2 :-
order(Colour1, N1),
order(Colour2, N2),
{N1 < N2}.
Colour1 =< Colour2 :-
order(Colour1, N1),
order(Colour2, N2),
{N1 =< N2}.
Colour1 > Colour2 :-
order(Colour1, N1),
order(Colour2, N2),
{N1 > N2}.
Colour1 >= Colour2 :-
order(Colour1, N1),
order(Colour2, N2),
{N1 >= N2}.
Colour1 =:= Colour2 :-
{Colour1 == Colour2}.
Colour1 =\= Colour2 :-
{Colour1 \== Colour2}.
order(red, 1).
order(orange, 2).
order(yellow, 3).
order(green, 4).
order(blue, 5).
order(indigo, 6).
order(violet, 7).
:- end_object.
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