@Override
    public boolean hasNextBarrier() {
        this.processAllStarts();
        return NON_EMITTING_SEED != this.seed;
    }
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