public Supplier<E> getSeedSupplier() {
        return Optional.ofNullable(this.seedSupplier).orElse(this::generateSeedFromStarts);
    }
Gets the provided seed supplier or provides {@link #generateSeedFromStarts()}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/ReducingBarrierStep.java