    function transferFrom(address _from, address _to, uint256 _amount) onlyPayloadSize(3 * 32) public returns (bool success) {
        require(allowed[_from][msg.sender] >= _amount);
        allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_amount);
        doTransfer(_from, _to, _amount);
        return true;
    }