@Override
    public Traverser.Admin<E> processNextStart() {
        this.processAllStarts();
        if (this.seed == NON_EMITTING_SEED) throw FastNoSuchElementException.instance();
        final Traverser.Admin<E> traverser = this.getTraversal().getTraverserGenerator().generate(this.generateFinalResult(this.seed), (Step<E, E>) this, 1l);
        this.seed = (E) NON_EMITTING_SEED;
        return traverser;
    }
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