if (fromVertex.equals(toVertex)) {
            return;
        }

        if (fromProject != null) {
            MavenProject toProject = projectMap.get(toVertex.getLabel());
            fromProject.addProjectReference(toProject);
        }

        if (force && toVertex.getChildren().contains(fromVertex)) {
            graph.removeEdge(toVertex, fromVertex);
        }

        try {

            return Collections.emptyList();
        } else {
            return Collections.unmodifiableList(build.getExtensions());
        }
    }

    public void addProjectReference(MavenProject project) {
        projectReferences.put(
                getProjectReferenceId(project.getGroupId(), project.getArtifactId(), project.getVersion()), project);
    }