def METHOD_NAME(self) -> typing.Tuple[str, sexp.Sexp]:
    """ E.popitem() -> (k, v), remove and return some (key, value)
    pair as a 2-tuple; but raise KeyError if E is empty. """
    if len(self) == 0:
        raise KeyError()
    kv = next(self.items())
    del self[kv[0]]
    return kv