@Override
        public <V> VertexProperty<V> property(final String key, final V value) {
            if (state.equals(State.MAP_REDUCE))
                throw GraphComputer.Exceptions.vertexPropertiesCanNotBeUpdatedInMapReduce();
            if (!computeKeys.contains(key))
                throw GraphComputer.Exceptions.providedKeyIsNotAnElementComputeKey(key);
            return new ComputerVertexProperty<>(this.getBaseVertex().property(key, value));
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/util/ComputerGraph.java