}

    @Override
    public NavigableSet<Node> getMustSuccessors() {
        return ImmutableSortedSet.of();
    }

    @Override
    public DependencyNodesSet addMustSuccessor(Node toNode) {
        return new ComplexDependencyNodesSet(this).addMustSuccessor(toNode);
    }

    /**
     * Notified when a node that is potentially a member of this set has completed.
     */
    @Override