protected E generateSeedFromStarts() {
        return starts.hasNext() ? projectTraverser(this.starts.next()) : null;
    }
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