@Override
    public void processAllStarts() {
        if (this.hasProcessedOnce && !this.starts.hasNext())
            return;
        this.hasProcessedOnce = true;

        if (this.seed == NON_EMITTING_SEED) {
            this.seed = getSeedSupplier().get();
        }

        while (this.starts.hasNext())
            this.seed = this.reducingBiOperator.apply(this.seed, this.projectTraverser(this.starts.next()));
    }
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