contract Ownable {
    /**
     * @dev Error constants.
     */
    string public constant NOT_CURRENT_OWNER = "018001";
    string public constant CANNOT_TRANSFER_TO_ZERO_ADDRESS = "018002";

    /**
     * @dev Current owner address.
     */
    address public owner;

    /**
     * @dev An event which is triggered when the owner is changed.
     * @param previousOwner The address of the previous owner.
     * @param newOwner The address of the new owner.
     */
    event OwnershipTransferred(
        address indexed previousOwner,
        address indexed newOwner
    );

    /**
     * @dev The constructor sets the original `owner` of the contract to the sender account.
     */
    constructor() {
        owner = msg.sender;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(msg.sender == owner, NOT_CURRENT_OWNER);
        _;
    }

    /**
     * @dev Allows the current owner to transfer control of the contract to a newOwner.
     * @param _newOwner The address to transfer ownership to.
     */
    function transferOwnership(address _newOwner) public onlyOwner {
        require(_newOwner != address(0), CANNOT_TRANSFER_TO_ZERO_ADDRESS);
        emit OwnershipTransferred(owner, _newOwner);
        owner = _newOwner;
    }
}
/**
 * @dev The contract has an owner address, and provides basic authorization control whitch
 * simplifies the implementation of user permissions. This contract is based on the source code at:
 * https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/ownership/Ownable.sol
 */
function transferOwnership(address _newOwner) public onlyOwner {
        require(_newOwner != address(0), CANNOT_TRANSFER_TO_ZERO_ADDRESS);
        emit OwnershipTransferred(owner, _newOwner);
        owner = _newOwner;
    }
/**
 * @dev Allows the current owner to transfer control of the contract to a newOwner.
 * @param _newOwner The address to transfer ownership to.
 */