%-- ­á ª, ó ¦ ÔÔñ½ ãØ·ãÆÖÒ á„¢¨á
a(1,2).
a(2,3).
a(4,3).
a(2,5).
a(5,6).
a(1,6).
a(4,5).

not_in(_,[]).
not_in(X,[X|_]):-!,fail.
not_in(X,[_|A]):-not_in(X,A).

len([],0).
len([_|B],I):-len(B,K),I is K+1.

%-- ÝáÖë¨áÆ  ãÒ¨éÔÖãå· ë¨áõ·Ô
g(X,Y):-a(X,Y).
g(X,Y):-a(Y,X).

%-- ÝÖ·ãÆ Øçå· ë ¬á ª¨ ë ¬Ð碷Ôç
%-- P(X,Y,P,V), ¬¦¨
%-- X - Ô û ÐíÔ Þ ë¨áõ·Ô , Y - ÆÖÔ¨ûÔ Þ ë¨áõ·Ô , P - Øçåí,
%-- V - ãÖ¦¨áé·å ãØ·ãÖÆ ó ¦¨½ãåëÖë ÔÔñµ ë¨áõ·Ô (÷å· ë¨áõ·Ôñ Ô¨ ¢ç¦çå
%-- ØÖëåÖáÔÖ ·ãØÖÐíóÖë åíãÞ Øá· ØÖ·ãƨ)

p(X,X,[X],V):-not_in(X,V).
p(X,Y,[X|P],V):-not_in(X,V),g(X,Z),p(Z,Y,P,[X|V]).

%-- ÝÖ·ãÆ ¤·ÆÐÖë ó ¦ ÔÔÖ½ ¦Ð·ÔÔñ, Øá· N=ÆÖÐ-ëÖ ë¨áõ·Ô
%-- ØÖÐçû·Ò ­ Ò·ÐíåÖÔÖë ¤·ÆÐ

cycle(Y,N):-len(Y,N),p(X,Z,Y,[]),g(Z,X).

%-- ô ØçãÆ
%-- consult(['g.pro']).
%-- p(1,6,X,[]).
%-- cycle(Y,6).