这个是关于Dynamic hedging定义的问题，Notes P82页，"The goal of a delta-hedge portfolio(or delta-neutral hedge)is to combine a long postion in a stock with a short postion in a call option so that the value of the portfolio does not change when the value of the stock changes."指的是我们已经有stock的头寸了，然后通过short call的方法来动态对冲stock价格下跌的风险。stock和call之间的数量关系就是用delta来衡量的。
