code: def _add_reference(reference, statement):
      '''
      Adds a reference to statement.
      '''
      type_, value = _expand_one_key_dictionary(reference)
      opt = Option(type_)
      param = SimpleParameter(value)
      opt.add_parameter(param)
      statement.add_child(optf _add_reference(reference, statement):
      '''
      Adds a reference to statement.
      '''
      type_, value = _expand_one_key_dictionary(reference)
      opt = Option(type_)
      param = SimpleParameter(value)
      opt.add_parameter(param)
      statement.add_child(opt)
docs: Adds a reference to statement.