how to make this fail driven loop succesfull ?

Hello, 


I have these facts: 


male(roelof). 

male(mans).

male(jan).


female(chantal). 

female(tamara). 

female(marie).

female(gerda). 


parent(roelof,tamara).

parent(mans, roelof). 

parent(jan, chantal). 

parent(marie, roelof). 

parent(chantal, tamara).

parent(mans, gerda).

parent(marie, gerda). 


is_mother(Child,Mother):- 

parent(Mother, Child),

female(Mother).



is_father(Child, Father):- 

parent(Father, Child),

male(Father). 



siblings(Father,Mother) :- 

is_father(X, Father), 

is_mother(X, Mother),

 write(X),

 nl, 

fail.


When I do siblings(mans,marie) it produces:


roelof

gerda

no


How can I change this to make the output like this:


roelof

gerda

yes. 


I have tried to add siblings(_) but with no success 


Roelof