@Override
    public void close() {
        CloseableIterator.closeIterator(iterator);
    }
Attempts to close an underlying iterator if it is of type {@link CloseableIterator}. Graph providers may choose
to return this interface containing their vertices and edges if there are expensive resources that might need to
be released at some point.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/GraphStep.java