public Collection<Traverser.Admin<S>> get(final I k) {
        final Collection<Traverser.Admin<S>> c = index.get(k);

        // if remove() is called on this class, then the MultiValueMap *may* (javadoc wasn't clear
        // what the expectation was - used the word "typically") return an empty list if the last
        // item removed leaves the list empty. i think we want to enforce null for TraverserSet
        // semantics
        return c != null && c.isEmpty() ? null : c;
    }
Gets a collection of {@link Traverser} objects that contain the specified value.

@param k the key produced by the indexing function
@return
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/traverser/util/IndexedTraverserSet.java