parents(william, diana,     charles).
parents(henry,   diana,     charles).
parents(charles, elizabeth, philip).
parents(diana,   frances,   edward).

parents(anne,    elizabeth, philip).
parents(andrew,  elizabeth, philip).
parents(edward,  elizabeth, philip).

married(diana,   charles).
married(elizabeth, philip).
married(frances, edward).
married(anne,    mark).

parent(C,M) :- parents(C,M,D).
parent(C,D) :- parents(C,M,D).

sibling(X,Y) :- parents(X,M,D), parents(Y,M,D). 

aORuDirect(C, A) :- parent(C,P) , sibling(P,A).
aORuMarr(C, A)   :- aORuDirect(C,X) , married(X,A).
aORuMarr(C, A)   :- aORuDirect(C,X) , married(A,X).
aORu(C,A)        :- aORuDirect(C,A).
aORu(C,A)        :- aORuMarr(C,A).


/*  Try each of the following queries separately.

married(diana, X).

aORu(william,X).

sibling(william,X).

*/