@Override
    public void resetBarrierFromValueTraversal() {
        this.barrierStep = determineBarrierStep(this.valueTraversal);
        this.getTraversal().getSideEffects().registerIfAbsent(this.sideEffectKey, HashMapSupplier.instance(),
                new GroupStep.GroupBiOperator<>(null == this.barrierStep ?
                        Operator.assign :
                        this.barrierStep.getMemoryComputeKey().getReducer()));
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/sideEffect/GroupSideEffectStep.java