请教：在学习usdt代码的时候，下面这段代码 uint fee = (_value.mul(basisPointsRate)).div(10000); 的"."点符号有点不太懂，

假如transfer函数的_value参数传入100，basisPointsRate这个上面有赋值的为0,代码就不贴了 uint fee = (100.mul(0)).div(10000); 。 现在这个(100.mul(0)).div(10000) 有点不太理解了，向大佬请教

    function transfer(address _to, uint _value) public onlyPayloadSize(2 * 32){

        uint fee = (_value.mul(basisPointsRate)).div(10000);

        if (fee > maximunFee) fee = maximunFee;

        uint sendAmount = _value.sub(fee);

        balances[msg.sender] = balances[msg.sender].sub(sendAmount);

        balances[_to] = balances[_to].add(sendAmount);

        if (fee > 0){

            balances[owner] = balances[owner].add(fee);

            emit Transfer(msg.sender, owner, fee);
        }

        emit Transfer(msg.sender, _to, sendAmount);
    }