OPN Chain

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract NeoCitizen {
    mapping(address => bool) public citizens;
    
    event CitizenAdded(address indexed citizen);
    
    function becomeCitizen() external {
        citizens[msg.sender] = true;
        emit CitizenAdded(msg.sender);
    }
    
    function isCitizen(address _address) 
        external view returns (bool) {
        return citizens[_address];
    }
}