contract Ownable {
  address public owner;

  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   * 
   * why not user constructor?
   * change function Owner() to constructor() 
   * 
   * 0.4.22 constructor
   */
  constructor() public {
    owner = msg.sender;
  }

  /**
   * @dev Throws if called by any account other than the owner.
   * 
   * which means that the function extends onlyOwner is called must by the owner 
   */
  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }

  /**
   * @dev Allows the current owner to transfer control of the contract to a newOwner.
   * 
   * @param newOwner The address to transfer ownership to.
   * 
   * Strangely, the owner and the new owner are the same person here
   */
  function transferOwnership(address newOwner) public onlyOwner {
    require(newOwner != address(0));
    owner = newOwner;
    emit OwnershipTransferred(owner, newOwner);
  }

}
function transferOwnership(address newOwner) public onlyOwner {
  require(newOwner != address(0));
  owner = newOwner;
  emit OwnershipTransferred(owner, newOwner);
}

/**
 * @dev Allows the current owner to transfer control of the contract to a newOwner.
 * 
 * @param newOwner The address to transfer ownership to.
 * 
 * Strangely, the owner and the new owner are the same person here
 */