private void assertCommonA(final Traversal<Vertex, Map<String, Collection<Vertex>>> traversal) {
        final Map<String, Collection<Vertex>> map = traversal.next();
        assertEquals(6, map.size());
        map.forEach((key, values) -> {
            assertEquals(1, values.size());
            assertEquals(convertToVertexId(key), values.iterator().next().id());
        });
        assertFalse(traversal.hasNext());
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-test/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/GroupTest.java