@Override
    public E nextBarrier() {
        if (!this.hasNextBarrier())
            throw FastNoSuchElementException.instance();
        else {
            final E temp = this.seed;
            this.seed = (E) NON_EMITTING_SEED;
            return temp;
        }
    }
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