@Override
    public <U> Property<U> property(final String key, final U value) {
        throw Element.Exceptions.propertyAdditionNotSupported();
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/util/reference/ReferenceVertexProperty.java