Siblings of parents

Hello, 


I have this facts: 


male(roelof). 

male(mans).

male(jan).


female(chantal). 

female(tamara). 

female(marie). 


parent(roelof,tamara).

parent(mans, roelof). 

parent(jan, chantal). 

parent(marie, roelof). 

parent(chantal, tamara). 


mother(Child):- 

parent(X, Child),

female(X),



father(Child):- 

parent(X, Child),

male(X),





siblings(F,M) :- 

father(X) 

mother(X).



So I thougth to display if parents have siblings X have to have a father and a mother. 


But I see error messages when I try to run this.


Why ?


Roelof