docs: def index(self, key, default=UNSET):
          """Find the first key-value pair with key *key* and return its position.
  
          If the key is not found, return *default*. If default was not provided,
          raise a ``KeyError``
          """
          self._find_lt(key)
          node = self._path[0][2]
          if node is self._tail or key < node[0]:
              if default is self.UNSET:
                  raise KeyError('key {!r} not in list'.format(key))
              return default
          return self._distance[0]