public List<String> getSelectKeys() {
        return this.selectKeys;
    }
Get the keys for this SelectStep. Unlike {@link SelectStep#getScopeKeys()}, this returns a list possibly with
a duplicate key. This guarantees to return the keys in the same order as passed in.
TODO: getScopeKeys should return order-aware data structure instead of HashSet so that graph providers can
      get the keys in the order passed in a query, and can associate them with by-traversals in a correct sequence.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/SelectStep.java