@Override
    protected Traverser.Admin<E> processNextStart() throws NoSuchElementException {
        final Traverser.Admin<S> traverser = this.starts.next();
        final Iterator<E> iterator = TraversalUtil.applyAll(traverser, this.mapTraversal);
        return  iterator.hasNext() ? traverser.split(iterator.next(), this) : EmptyTraverser.instance();
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/TraversalMapStep.java