Add or remove token to/from list.

        If token is in this list, the token will be removed. Otherwise add it
        to list.

def toggle(self, token: str) -> None:
        """Add or remove token to/from list.

        If token is in this list, the token will be removed. Otherwise add it
        to list.
        """
        self._validate_token(token)
        if token in self:
            self.remove(token)
        else:
            self.add(token)