@Override
    public Set<String> getScopeKeys() {
        // can't return scope keys here because they aren't known prior to traversal execution and this method is
        // used at strategy application time. not getting any test failures as a result of returning empty. assuming
        // that strategies don't use Scoping in a way that requires the keys to be known and if they aren't doesn't
        // hose the whole traversal. in the worst case, strategies will hopefully just leave steps alone rather than
        // make their own assumptions that the step is not selecting anything. if that is happening somehow we might
        // need to modify Scoping to better suite this runtime evaluation of the key.
        return Collections.emptySet();
    }
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/TraversalSelectStep.java