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