@Override
    protected E map(final Traverser.Admin<S> traverser) {
        //noinspection unchecked
        return (E) indexer.apply(IteratorUtils.asIterator(traverser.get()));
    }
Gets the type of indexer that is configured for this step.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/IndexStep.java