private static final String ERROR_NEGATIVE_COUNT = "is non-negative";
  static final String ERROR_SELF_LOOP = "self-loops are not allowed";

  @Test
  public void transitiveClosure_directedGraph() {
    MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(false).build();
    directedGraph.putEdge(N1, N2);
    directedGraph.putEdge(N1, N3);
    directedGraph.putEdge(N2, N3);