private static boolean objectsEqual(Object o1, Object o2) {
        if (o1 == null && o2 == null) {
            return true;
        }
        if (o1 == null || o2 == null) {
            return false; // as they are not both null
        }
        return o1.equals(o2);
    }

    // ----------------------------------------------------------------------------
    /**
     * used to eliminate exact duplicates in the edge list
     */