text: from surround import Estimator, State, Stage
  
  
  class AssemblyState(State):
      input_data = None
      output_data = ""
      lines = []
  
  class InputValidator(Stage):
      def operate(self, state, config):
          if not state.input_data:
              raise ValueError("'input_data' is None")
  
  
  class HelloWorld(Estimator):
      def estimate(self, state, config):
          state.output_data += state.input_data
  
      def fit(self, state, config):
          print("TODO: Building models")
  