public void setReducingBiOperator(final BinaryOperator<E> reducingBiOperator) {
        this.reducingBiOperator = reducingBiOperator;
    }
If the {@code seedSupplier} is {@code null} then this method is called.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/ReducingBarrierStep.java