code: def output_reference(self, name):
          
          if name not in self.output_names:
              raise ValueError('Invalid output "{}"'.format(name))
          return Reference(step_name=self.name_in_workflow, output_name=name)
docs: Return a reference to the given output for use in an input
              of a next Step.
  
          For a Step named `echo` that has an output called `echoed`, the
          reference `echo/echoed` is returned.
  
          Args:
              name (str): the name of the Step output
          Raises:
              ValueError: The name provided is not a valid output name for this
                  Step.