@Test
        @LoadGraphWith(LoadGraphWith.GraphData.CREW)
        public void shouldNotWriteGraphMLFromGraphWithMultiProperties() throws Exception {
            try (ByteArrayOutputStream bos = new ByteArrayOutputStream()) {
                final GraphMLWriter w = GraphMLWriter.build().create();
                w.writeGraph(bos, graph);
                fail("Should not be able to write multi-properties to GraphML");
            } catch (IllegalStateException iae) {
                assertEquals(iae.getMessage(), "Multiple properties exist for the provided key: [location]. Multi-properties are not directly supported by GraphML format");
            }
        }
Only need to execute this test with TinkerGraph or other graphs that support user supplied identifiers.
gremlin-test/src/main/java/org/apache/tinkerpop/gremlin/structure/io/IoTest.java