Write prolog query: closest_common_ancestor.

x is a closest common ancestor of two people y and z if x is an ancestor of both y and z and no child of x is an ancestor of y and z. (A closest common ancestor of two ?rst cousins will be someone who is a grandparent of both.)

first_cousin(C1,C2):- parent(P1,C1), parent(P2,C2), sibling(P1,P2).

grand_parent(GP,GC):- parent(GP,P), parent(P,GC).