private void setValueTraversal(final Traversal.Admin valueTraversal) {
        this.valueTraversal = this.integrateChild(convertValueTraversal(valueTraversal));
        this.barrierStep = determineBarrierStep(this.valueTraversal);
        this.getTraversal().getSideEffects().register(this.sideEffectKey, null,
                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