docs: def _GenConfig(self, cfg):
      """Interpolate configurations with defaults to generate actual configs."""
      # Some setting names may have a + or - suffix. These indicate that the
      # settings modify the default values.
      merged = self.default.copy()
      for setting, vals in iteritems(cfg):
        option, operator = (setting.split(None, 1) + [None])[:2]
        vals = set(vals)
        default = set(self.default.get(option, []))
        # If there is an operator, updated values accordingly.
        if operator == "+":
          vals = default.union(vals)
        elif operator == "-":
          vals = default.difference(vals)
        merged[option] = list(vals)
      return rdf_protodict.AttributedDict(**merged)