Prolog 序言中的双胞胎将重复答案

假设我有3个孩子

person(pet,fox,date(5,may,2004),unemployed).  
person(jim,fox,date(5,may,2004),unemployed).  
person(george,fox,date(9,december,2002),unemployed).  

child(X):-X=person(pet,fox,date(5,may,2004),unemployed).  
child(X):-X=person(jim,fox,date(5,may,2004),unemployed).  
child(X):-X=person(george,fox,date(9,december,2002),unemployed).  

我想要这些孩子的双胞胎

twins(Child1,Child2) :-
    child(Child1),child(Child2),
    Child1=person(Fname1,_,Date1,_),
    Child2=person(Fname2,_,Date2,_),
    Fname1\=Fname2,
    Date1=Date2.

问题是当我问prolog twins（X，Y）时。我会再得到一些答案。如何预防

?- twins(X,Y).  
X = person(pet, fox, date(5, may, 2004), unemployed),  
Y = person(jim, fox, date(5, may, 2004), unemployed) ;  
X = person(jim, fox, date(5, may, 2004), unemployed),  
Y = person(pet, fox, date(5, may, 2004), unemployed) ;  
false.  

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您可以通过对该对的元素施加非对称顺序来防止生成所有置换：

?- twins(X, Y), X @< Y.
X = person(jim, fox, date(5, may, 2004), unemployed),
Y = person(pet, fox, date(5, may, 2004), unemployed) ;
false.

child(X) :-
    person(FirstName, LastName, DOB, Status),
    X = person(FirstName, LastName, DOB, Status).