function mint(address _to, uint256 _amount, bytes _operatorData) public onlyOwner {
        require (totalSupply.add(_amount) <= maxSupply);
        requireMultiple(_amount);

        totalSupply = totalSupply.add(_amount);
        balanceOf[_to] = balanceOf[_to].add(_amount);

        callRecipient(msg.sender, 0x0, _to, _amount, "", _operatorData, true);

        emit Minted(msg.sender, _to, _amount, _operatorData);
        emit Transfer(0x0, _to, _amount);
    }