@Test
    public void shouldPutValueInExistingSet() {
        final Map<String,Set<Object>> multi = new HashMap<>();
        MultiMap.put(multi, "here", 1);
        MultiMap.put(multi, "there", 3);

        final Set<Object> expected1 = new HashSet<Object>() {{
            add(1);
        }};
        assertEquals(expected1, multi.get("here"));

        MultiMap.put(multi, "here", 2);

        final Set<Object> expected2 = new HashSet<Object>() {{
            add(1);
            add(2);
        }};
        assertEquals(expected2, multi.get("here"));
    }
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/util/MultiMapTest.java