private void addValue(final Vertex vertex, final String key, final VertexProperty property) {
        final Map<String, List<VertexProperty<?>>> elementProperties = this.computeProperties.computeIfAbsent(vertex, k -> new HashMap<>());
        elementProperties.compute(key, (k, v) -> {
            if (null == v) v = new ArrayList<>();
            v.add(property);
            return v;
        });
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
tinkergraph-gremlin/src/main/java/org/apache/tinkerpop/gremlin/tinkergraph/process/computer/TinkerGraphComputerView.java