@Override
            public VertexProperty.Cardinality getCardinality(final String key) {
                // probably not much hurt here in returning list...it's an "empty graph"
                return VertexProperty.Cardinality.list;
            }
Vertex features defined such that they support immutability but allow all other possibilities.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/util/empty/EmptyGraph.java