docs: def _FlattenToCallback(tokens, callback):
      """Takes a nested list structure and flattens it.
  
    ['a', ['b', 'c']] -> callback('a'); callback('b'); callback('c');
    """
      for t in tokens:
          if isinstance(t, six.string_types):
              callback(t)
          else:
              _FlattenToCallback(t, callback)