    function revokeOperator(address operator) external {
        require(operator != msg.sender, "ERC777: revoking self as operator");
        if (_defaultOperators[operator]) {
            _revokedDefaultOperators[msg.sender][operator] = true;
        } else {
            delete _operators[msg.sender][operator];
        }
        emit RevokedOperator(operator, msg.sender);
    }