code: def set_root(self, op):
      self.root_op = op
      return self
  
file_path: ReAgent/serving/reagent/serving/config/builder.py
imports: from typing import Any, Dict
  import reagent.serving.config.config
  import reagent.serving.config.namespace
  import reagent.serving.config.operators
  from reagent.serving.config.serialize import config_to_json
label_desc: Use image node_id 2 for calling the DecisionPlanBuilder obj's underlying member method code with example usage: obj.set_root(op) and returns: self