-/* Prolog test file */
 -male(john).
 -male(peter).
 -
 -female(vick).
 -female(christie).
 -
 -parents(john, peter, christie).
 -parents(vick, peter, christie).
 -
 -/* X is a brother of Y */
 -brother(X, Y) :- male(X), parents(X, F, M), parents(Y, F, M).