private String pullOutVariableStartStepToParent(final WhereTraversalStep<?> whereStep) {
        return this.pullOutVariableStartStepToParent(new HashSet<>(), whereStep.getLocalChildren().get(0), true).size() != 1 ? null : pullOutVariableStartStepToParent(new HashSet<>(), whereStep.getLocalChildren().get(0), false).iterator().next();
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MatchStep.java