docs: def eval_extensions(self, value, name, option, format):
          """Evaluates extensions in the registry.  If some extension handles the
          format string, it returns a string.  Otherwise, returns ``None``.
          """
          try:
              exts = self._extensions[name]
          except KeyError:
              raise ValueError('no suitable extension: %s' % name)
          for ext in exts:
              rv = ext(self, value, name, option, format)
              if rv is not None:
                  return rv