abstract contract Burnable is Context {

  mapping(address => bool) private _burners;

  event BurnerAdded(address indexed account);
  event BurnerRemoved(address indexed account);

  /**
    * @dev Returns whether the address is burner.
    */
  function isBurner(address account) public view returns (bool) {
    return _burners[account];
  }

  /**
    * @dev Throws if called by any account other than the burner.
    */
  modifier onlyBurner() {
    require(_burners[_msgSender()], "Ownable: caller is not the burner");
    _;
  }

  /**
    * @dev Add burner, only owner can add burner.
    */
  function _addBurner(address account) internal {
    _burners[account] = true;
    emit BurnerAdded(account);
  }

  /**
    * @dev Remove operator, only owner can remove operator
    */
  function _removeBurner(address account) internal {
    _burners[account] = false;
    emit BurnerRemoved(account);
  }
}
/**
 * @dev Extension of {ERC20} that allows token holders to destroy both their own
 * tokens and those that they have an allowance for, in a way that can be
 * recognized off-chain (via event analysis).
 */
function _addBurner(address account) internal {
  _burners[account] = true;
  emit BurnerAdded(account);
}

/**
 * @dev Add burner, only owner can add burner.
 */