@Override
        public Set<String> getScopeKeys() {
            return null == this.matchKey ? Collections.emptySet() : Collections.singleton(this.matchKey);
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/filter/WhereTraversalStep.java