Unfold a returned object into a linear form.
unfold() step is a flatMap step.
Unfold a returned object into a linear form, such as a map into each individual value.
Find the top three countries that the most people have lived in:
g.V().hasLabel('person').values('country'). groupCount(). order(local). by(values, decr). limit(local, 3). select(keys).unfold()