@Override
    public void writeVertices(final OutputStream outputStream, final Iterator<Vertex> vertexIterator, final Direction direction) throws IOException {
        throw Io.Exceptions.writerFormatIsForFullGraphSerializationOnly(this.getClass());
    }
This method is not supported for this writer.

@throws UnsupportedOperationException when called.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/graphml/GraphMLWriter.java