@Override
    public Set<String> keys() {
        return null == this.properties ? Collections.emptySet() : this.properties.keySet();
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
tinkergraph-gremlin/src/main/java/org/apache/tinkerpop/gremlin/tinkergraph/structure/TinkerEdge.java