code: def add(self, name: str, value: str) -> None:
          
          norm_name = _normalized_headers[name]
          self._last_key = norm_name
          if norm_name in self:
              self._dict[norm_name] = (
                  native_str(self[norm_name]) + "," + native_str(value)
              )
              self._as_list[norm_name].append(value)
          else:
              self[norm_name] = value
docs: Adds a new value for the given key.