where do I think wrong

Hello, 

I have these clauses 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).

is_father(F,C) :- 
   parent(F,C), male(F).

is_mother(M,C) :-
   parent(M,C),female(M).

have_children(M,F) :-
   is_mother(M,Z), 
   is_father(F,Z).

display_mother(C):-
   is_mother(M,C), 
   write('De moeder van '), 
   write(C) , 
   write(' is '),
   write(M).

display_father(C) :-
   is_father(F,C), 
   write('De vader van '),
   write(C),
   write(' is '),
   write(F).

list_parents(C) :-
   parent(X,C), 
   tab(5), 
   write(X),
   nl, 
   fail.
   list_parents(_).

display_parents(C) :-
   write('De ouders van '), 
   write(C), 
   write(' zijn :'),
   nl, 
   nl,
   list_parents(C).
   display_parents(_).

have_siblings(C) :-
    is_father(X,C),
    is_father(X,S), 
    is_mother(Y,S), 
    is_mother(Y,C), 
    C \= S,
    write(S).
    

display_siblings(C) :-
   write('De broers/zusters van '), 
   write(C), 
   write(' zijn :'),
   nl, 
   nl,
   have_siblings(C).
   display_siblings(_).


It worked fine if a mother and father are known.
So I want to change it to this :

have_siblings(C) :-
    is_father(X,C),
    is_father(X,S); 
    is_mother(Y,S), 
    is_mother(Y,C), 
    C \= S,
    write(S).

So if a father of a child(C)  is equal to the father of a sibling(S) or the mother of a child is equal to the mother of a sibling.
But nothing gets found then. 

Here is a trace : 

  Call: (6) display_siblings(chantal) ? creep
   Call: (7) write('De broers/zusters van ') ? creep
De broers/zusters van 
   Exit: (7) write('De broers/zusters van ') ? creep
   Call: (7) write(chantal) ? creep
chantal
   Exit: (7) write(chantal) ? creep
   Call: (7) write(' zijn :') ? creep
 zijn :
   Exit: (7) write(' zijn :') ? creep
   Call: (7) nl ? creep

   Exit: (7) nl ? creep
   Call: (7) nl ? creep

   Exit: (7) nl ? creep
   Call: (7) have_siblings(chantal) ? creep
   Call: (8) is_father(_G1969, chantal) ? creep
   Call: (9) parent(_G1969, chantal) ? creep
   Exit: (9) parent(jan, chantal) ? creep
   Call: (9) male(jan) ? creep
   Exit: (9) male(jan) ? creep
   Exit: (8) is_father(jan, chantal) ? creep
   Call: (8) is_father(jan, _G1970) ? creep
   Call: (9) parent(jan, _G1970) ? creep
   Exit: (9) parent(jan, chantal) ? creep
   Call: (9) male(jan) ? creep
   Exit: (9) male(jan) ? creep
   Exit: (8) is_father(jan, chantal) ? creep
   Exit: (7) have_siblings(chantal) ? creep
   Exit: (6) display_siblings(chantal) ? creep
true .

Roelof

 		 	   		  
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