private void setValueTraversal(final Traversal.Admin kvTraversal) {
        this.valueTraversal = this.integrateChild(convertValueTraversal(kvTraversal));
        this.barrierStep = determineBarrierStep(this.valueTraversal);
        this.setReducingBiOperator(new 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/map/GroupStep.java