function _transfer(address _from, address _to, uint256 _value) internal {
        _preValidTransfer(_from, _to, _value);

        uint256 previousBalances = _balanceOf[_from] + _balanceOf[_to];

        _sendToken(_from, _to, _value);

        assert(_balanceOf[_from] + _balanceOf[_to] == previousBalances);
    }