docs: def iter_multi_items(mapping):
      """Iterates over the items of a mapping yielding keys and values
      without dropping any from more complex structures.
      """
      if isinstance(mapping, MultiDict):
          for item in iteritems(mapping, multi=True):
              yield item
      elif isinstance(mapping, dict):
          for key, value in iteritems(mapping):
              if isinstance(value, (tuple, list)):
                  for value in value:
                      yield key, value
              else:
                  yield key, value
      else:
          for item in mapping:
              yield item