docs: def _parse_arg(func, variables, arg_name, anno):
      """Parse an argument's annotation."""
      if isinstance(anno, str):
          var = variables[anno]
          return var, var.read_latest
      elif (isinstance(anno, list) and len(anno) == 1 and
            isinstance(anno[0], str)):
          var = variables[anno[0]]
          return var, var.read_all
      # For now, be very strict about annotation format (e.g.,
      # allow list but not tuple) because we might want to use
      # tuple for other meanings in the future.
      raise StartupError(
          'cannot parse annotation %r of parameter %r for %r' %
          (anno, arg_name, func))