@Override
    public boolean add(final Traverser.Admin<S> traverser) {
        final boolean newOne = super.add(traverser);

        // if newly added then the traverser will be the same as the one passed in here to add().
        // if it is not, then it was merged to an existing traverser and the bulk would have
        // updated on that reference, thus only new stuff really needs to be added to the index
        if (newOne) index.put(indexingFunction.apply(traverser.get()), traverser);

        return newOne;
    }
A {@link TraverserSet} that has an index back to the object in the {@link Traverser}. Using this extension of
{@link TraverserSet} can make it easier to find traversers within the set if the internal value is known. Without
the index the entire {@link TraverserSet} needs to be iterated to find a particular value.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/traverser/util/IndexedTraverserSet.java