@Override
    public ReducingBarrierStep<S, E> clone() {
        final ReducingBarrierStep<S, E> clone = (ReducingBarrierStep<S, E>) super.clone();
        clone.hasProcessedOnce = false;
        clone.seed = (E) NON_EMITTING_SEED;
        return clone;
    }
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