code: def reftag_to_cls(fn):
      
      names, _, _, values = inspect.getargspec(fn)
      @wraps(fn)
      def wrapped(*args, **kwargs):
          i = 0
          backend = args[0]
          for name in names[1:]:
              value = args[i]
              if name == "concrete" and isinstance(value, six.string_types):
                  args[i] = backend.REFTAG_CONCRETE[value]
              elif name == "resource" and isinstance(value, six.string_types):
                  args[i] = backend.REFTAG_RESOURCE[value]
              i += 1
          return fn(*args, **kwargs)
      return wrapped
docs: decorator that checks function arguments for `concrete` and `resource`
      and will properly set them to class references if a string (reftag) is
      passed as the value