code: def _is_reference(arg):
      '''
      Return True, if arg is a reference to a previously defined statement.
      '''
      return isinstance(arg, dict) and len(arg) == 1 and isinstance(next(six.itervalues(arg)), six.string_typesf _is_reference(arg):
      '''
      Return True, if arg is a reference to a previously defined statement.
      '''
      return isinstance(arg, dict) and len(arg) == 1 and isinstance(next(six.itervalues(arg)), six.string_types)
docs: Return True, if arg is a reference to a previously defined statement.