    function burnFrom(address _from, uint256 _value) public returns(bool success) {
        require(balanceOf[_from] >= _value); 
        require(_value <= allowance[_from][msg.sender]); 
        balanceOf[_from] -= _value; 
        allowance[_from][msg.sender] -= _value; 
        totalSupply -= _value; 
        emit Burn(_from, _value);
        return true;
    }