docs: def _conform_pair(self, pair):
          """Force a given key/value pair into a certain form.
          
          Override the _conform_key and _conform_value if you want to change
          the mapping behaviour.
          
          """
          pair = tuple(pair)
          if len(pair) != 2:
              raise ValueError('MultiMap element must have length 2')
          return (self._conform_key(pair[0]), self._conform_value(pair[1]))