public void setSeedSupplier(final Supplier<E> seedSupplier) {
        this.seedSupplier = seedSupplier;
    }
If the {@code seedSupplier} is {@code null} then the default behavior is to generate the seed from the starts.
This supplier must be callable as a constant and not rely on state from the class. Prefer overriding
{@link #generateSeedFromStarts()} otherwise.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/ReducingBarrierStep.java