public static Set<String> getStartLabels(final Traversal.Admin<Object, Object> traversal) {
            return ((Scoping) traversal.getStartStep()).getScopeKeys();
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MatchStep.java