graph.addVertex("a");
        assertEquals(1, graph.getVertices().size());
        assertEquals("a", graph.getVertex("a").getLabel());
        graph.addVertex("a");
        assertEquals(1, graph.getVertices().size());
        assertEquals("a", graph.getVertex("a").getLabel());

        graph.addVertex("b");
        assertEquals(2, graph.getVertices().size());
        assertFalse(hasEdge(graph, "a", "b"));

    private MetadataGraph findLinkedSubgraph(MetadataGraph g) {
        if (g.getVertices().size() == 1) {
            return g;
        }

        List<MetadataGraphVertex> visited = new ArrayList<>(g.getVertices().size());
        visit(g.getEntry(), visited, g);

        List<MetadataGraphVertex> dropList = new ArrayList<>(g.getVertices().size());

        // collect drop list

        assertNotNull(res, "null graph after resolver");
        assertNotNull(res.getVertices(), "no vertices in the resulting graph after resolver");

        assertNotNull(res.getExcidentEdges(v1), "no edges in the resulting graph after resolver");

        assertEquals(4, res.getVertices().size(), "wrong # of vertices in the resulting graph after resolver");
        assertEquals(
                2,

        VISITED
    }

    final Map<String, Vertex> vertices = new LinkedHashMap<>();

    public Vertex getVertex(String id) {
        return vertices.get(id);
    }

    public Collection<Vertex> getVertices() {
        return vertices.values();
    }

    Vertex addVertex(String label) {
        return vertices.computeIfAbsent(label, Vertex::new);
    }

        protected ClasspathGraphVisitor(MetadataGraph cleanGraph, ClasspathContainer cpc) {
            this.cpc = cpc;
            this.graph = cleanGraph;

            visited = new ArrayList<>(cleanGraph.getVertices().size());
        }

        // -----------------------------------------------------------------------
        protected void visit(MetadataGraphVertex node) {
            ArtifactMetadata md = node.getMd();

        VISITED
    }

    final Map<String, Vertex> vertices = new LinkedHashMap<>();

    public Vertex getVertex(String id) {
        return vertices.get(id);
    }

    public Collection<Vertex> getVertices() {
        return vertices.values();
    }

    Vertex addVertex(String label) {
        return vertices.computeIfAbsent(label, Vertex::new);
    }