contract Ownable is IOwnableEvents {
    address public owner;
    address private nextOwner;

    // modifiers

    modifier onlyOwner() {
        require(isOwner(), "caller is not the owner.");
        _;
    }

    modifier onlyNextOwner() {
        require(isNextOwner(), "current owner must set caller as next owner.");
        _;
    }

    /**
     * @dev Initialize contract by setting transaction submitter as initial owner.
     */
    constructor(address owner_) {
        owner = owner_;
        emit OwnershipTransferred(address(0), owner);
    }

    /**
     * @dev Initiate ownership transfer by setting nextOwner.
     */
    function transferOwnership(address nextOwner_) external onlyOwner {
        require(nextOwner_ != address(0), "Next owner is the zero address.");

        nextOwner = nextOwner_;
    }

    /**
     * @dev Cancel ownership transfer by deleting nextOwner.
     */
    function cancelOwnershipTransfer() external onlyOwner {
        delete nextOwner;
    }

    /**
     * @dev Accepts ownership transfer by setting owner.
     */
    function acceptOwnership() external onlyNextOwner {
        delete nextOwner;

        owner = msg.sender;

        emit OwnershipTransferred(owner, msg.sender);
    }

    /**
     * @dev Renounce ownership by setting owner to zero address.
     */
    function renounceOwnership() external onlyOwner {
        _renounceOwnership();
    }

    /**
     * @dev Returns true if the caller is the current owner.
     */
    function isOwner() public view returns (bool) {
        return msg.sender == owner;
    }

    /**
     * @dev Returns true if the caller is the next owner.
     */
    function isNextOwner() public view returns (bool) {
        return msg.sender == nextOwner;
    }

    function _setOwner(address previousOwner, address newOwner) internal {
        owner = newOwner;
        emit OwnershipTransferred(previousOwner, owner);
    }

    function _renounceOwnership() internal {
        emit OwnershipTransferred(owner, address(0));

        owner = address(0);
    }
}
function cancelOwnershipTransfer() external onlyOwner {
    delete nextOwner;
}

/**
 * @dev Cancel ownership transfer by deleting nextOwner.
 */