why does this not display the right answer - prolog

Question

I have these facts and rules :
male(roelof).
male(mans).
male(ronald).
male(jan).
female(chantal).
female(marie).
female(gerda).
female(dagmar).
female(denise).
female(kimberly).
parent(mans,gerda).
parent(mans,roelof).
parent(marie,gerda).
parent(marie,roelof).
parent(dagmar,denise).
parent(dagmar,kimberly).
parent(ronald,denise).
parent(ronald,kimberly).
parent(chantal,tamara).
parent(roelof,tamara).
parent(jan,chantal).
parent(jan,dagmar).
father_child(Father, Child) :-
parent(Father, Child),
male(Father).
mother_child(Mother, Child) :-
parent(Mother, Child),
female(Mother).
child_father_mother(Child, Father, Mother) :-
father_child(Father, Child),
mother-child(Mother, Child).
same_father(Child, Sibling) :-
father_child(Father, Child),
father_child(Father, Sibling).
same_mother(Child, Sibling) :-
mother_child(Mother, Child),
mother_child(Mother, Sibling).
siblings(Child, Sibling) :-
same_father(Child, Sibling),
Child \= Sibling.
siblings(Child, Sibling) :-
same_mother(Child, Sibling),
Child \= Sibling.
display_siblings(X,Y) :-
setof(X-Y, (siblings(X,Y), \+X=Y), Sibs),
member((X-Y,Y), Sibs),
\+ (Y#<X, member((Y,X), Sibs)).
But when I do display_siblings I expected to see roelof-gerda.
But the output is only x=roelof
What did I do wrong.
Im a beginner at Prolog and try to understand how this works.
Roelof
Edit 1: Could this be a solution: How can I prevent duplicates in prolog
Edit 2: I will do but I still do not understand why 'setof(X-Y, (siblings(X,Y), X #< Y), Sibs),write(Sibs).' is wrong. It works in both cases display_'siblings(gerda,X)' and 'display_siblings(X,Y)'

Answer 1

Most of your problem lies in your display_siblings predicate:
display_siblings(X,Y) :-
setof(X-Y, (siblings(X,Y), \+X=Y), Sibs),
member((X-Y,Y), Sibs),
\+ (Y#<X, member((Y,X), Sibs)).
In the second line, the \+X=Y is superfluous since siblings already enforces that they are different. So that can be:
setof(X-Y, siblings(X,Y), Sibs),
The next line is querying whether (X-Y,Y) is a member of Sibs. However, Sibs consists of X-Y terms which will never match an (X-Y,Y) term. So this call to member will always fail. It should probably be:
member(X-Y, Sibs),
And then the last line is overly complex if all you want to do is eliminate symmetrical duplicates. You can simply use X #< Y and for efficiency, make this part of the setof check:
display_siblings(Sibs) :-
setof(X-Y, (siblings(X,Y), X #< Y), Sibs).
This will yield:
?- display_siblings(Sibs).
Sibs = [chantal-dagmar, denise-kimberly, gerda-roelof].
If you want to select all of the siblings for a given person and still avoid the symmetry redundancies, you may need to process out those redundancies after the setof:
display_siblings(X, Sibs) :-
setof(A-B, ((X = A ; X = B), siblings(A,B)), SibPairs),
pack(SibPairs, Sibs).
pack([X-Y|T], Sibs) :-
pack(T, SibList),
( member(Y-X, SibList)
-> SibList = Sibs
; Sibs = [X-Y|SibList]
).
pack([], []).
?- display_siblings(gerda, Sibs).
Sibs = [roelof-gerda].