How can I use the .io("filename.json").write() pattern to append to an existing graphson file?

 private static Graph buildGraph() {
        BaseConfiguration configuration = new BaseConfiguration();
        configuration.setProperty("gremlin.tinkerGraph.vertexIdManager", "ANY");

        return TinkerGraph.open(configuration);
    }

    @Test
    public void test() {
        Graph graph = buildGraph();
        GraphTraversalSource g;
        g = AnonymousTraversalSource.traversal().withEmbedded(graph);

        g.addV("source").property(T.id, "firstSource")
                .addV("sink").property(T.id, "firstSink")
                .addE("edge").property(T.id, "firstEdge").from(__.V("firstSource")).to(__.V("firstSink"))
                .iterate();

        g.addV("source").property(T.id, "secondSource")
                .addV("sink").property(T.id, "secondSink")
                .addE("edge").property(T.id, "secondEdge").from(__.V("secondSource")).to(__.V("secondSink"))
                .iterate();

        final Graph firstResult = (Graph) g.E("firstEdge").subgraph("subgraph").cap("subgraph").next();

        GraphTraversalSource firstGts = AnonymousTraversalSource.traversal().withEmbedded(firstResult);
        firstGts.io("src/test/resources/example.json").write().iterate();

        // Here the initial contents of the file are overwritten

        final Graph secondResult = (Graph) g.E("secondEdge").subgraph("subgraph").cap("subgraph").next();

        GraphTraversalSource secondGts = AnonymousTraversalSource.traversal().withEmbedded(secondResult);
        secondGts.io("src/test/resources/example.json").write().iterate();


    }

{"id":"secondSink","label":"sink","inE":{"edge":[{"id":"secondEdge","outV":"secondSource"}]}}
{"id":"secondSource","label":"source","outE":{"edge":[{"id":"secondEdge","inV":"secondSink"}]}}