例:假设A用户创建了订单要用10bit交换20eth, B用户只有5eth, B发起交易
feeMakeXfer = 5 * feeMake / 1 ether
feeTakeXfer = 5 * feeTake / 1 ether
A账户等级=0, feeRebateXfer = 0
A账户等级=1, feeRebateXfer = 5 * feeRebate / 1 ether
A账户等级=2, feeRebateXfer = feeTakeXfer
设置fee的时候要保证,feeRebateXfer <= feeTakeXfer
tokens[eth][B] = tokens[eth][B] - (5 + feeTakeXfer)
tokens[eth][A] = tokens[eth][A] + (5 + feeRebateXfer - feeMakeXfer )
tokens[eth][feeAccount] = tokens[eth][feeAccount] + (feeMakeXfer + feeTakeXfer - feeRebateXfer)
tokens[bit][A] = tokens[bit][A] - 5 * 10 / 20
tokens[bit][B] = tokens[bit][B] + 5 * 10 / 20