public List<FeatureRequirement> featuresRequired() {
            switch (this) {
                case SIMPLE:
                    return featuresRequiredBySimple;
                case VERTICES_ONLY:
                    return featuresRequiredByVerticesOnly;
            }

            throw new RuntimeException("No features for this GraphData type");
        }
Allows for adding of vertices (but not edges) with string property values.
gremlin-test/src/main/java/org/apache/tinkerpop/gremlin/FeatureRequirementSet.java