@Override
    public void addBarrier(final E barrier) {
        this.seed = NON_EMITTING_SEED == this.seed ?
                barrier :
                this.reducingBiOperator.apply(this.seed, barrier);
    }
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