let session = pl.create(1000);

let program = `
    parent(john, mary).
    parent(susan, mary).

    parent(mary, alice).
    parent(mary, bob).

    grandparent(X, Y) :- parent(X, Z), parent(Z, Y).
    sibling(X, Y) :- parent(Z, X), parent(Z, Y), X \\= Y.
    ancestor(X, Y) :- parent(X, Y).
    ancestor(X, Y) :- parent(X, Z), ancestor(Z, Y).
`;

 function showAnswer (a) {
   console.log(a)
 }

 session.consult(program, {
    success: function() {
        session.query("grandparent(X, bob).", {
            success: function(goal) {
                session.answers(showAnswer);
            } 
        }); 
    }
});