// user
struct User {
    uint UserListPointer; // needed to delete a "User"
    bytes32[] AccountKeys; // User has many "Account"s
    mapping(bytes32 => uint) AccountKeyPointers;
    //User properties
    bytes32 userName;
}

mapping(address => User) private Users;
address[] private UserList;

event UserRegistered_Successful(address userAddress, bytes32 userName);

/**
* @dev Function to check if _userAddress exist
* @param _userAddress the address to check if it exist.
* @return boolean flag if _userAddress exist.
*/
function isUser(address _userAddress) public view returns(bool isIndeed) {
    if(UserList.length==0) return false;
    return UserList[Users[_userAddress].UserListPointer]==_userAddress;
}

/**
* @dev Function to register User
* @param _userName the bytes32 to insert userName.
* @return boolean flag if register success.
*/
function registerUser(bytes32 _userName) public returns (bool isIndeed) {
    if(isUser(msg.sender)) revert(); // duplicate user prohibited
    Users[msg.sender].ListPointer = UserList.push(msg.sender)-1;
    Users[msg.sender].userName = _userName;

    UserRegistered_Successful(msg.sender, _userName);
    return true;
}