I'm studying Solidity and I couldn't understand what does [!] mean
What does it mean? And, how to use it?
Could you give any advice, please?
function sendReward(uint256 _reward, address _dest, uint256 _id) external
require(_reward > 0); //
require(address(this).balance >= _reward); //
require(_dest != address(0)); //
require(_dest != owner); //
rewardSent[_id] = true; //
emit RewardSent(_dest, _reward, _id);