public Object get() {
        return o;
    }
This class represents the state of the output of a child {@link Traversal} where it is either productive or not.
It is productive if it has at least one traverser within it, even if that traverser is holding a {@code null}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/util/TraversalProduct.java