code: def pop(self, key, default=_missing):
          
          try:
              return dict.pop(self, key)[0]
          except KeyError, e:
              if default is not _missing:
                  return default
              raise BadRequestKeyError(str(e))
docs: Pop the first item for a list on the dict.  Afterwards the
          key is removed from the dict, so additional values are discarded:
  
          >>> d = MultiDict({"foo": [1, 2, 3]})
          >>> d.pop("foo")
          1
          >>> "foo" in d
          False
  
          :param key: the key to pop.
          :param default: if provided the value to return if the key was
                          not in the dictionary.