何老师在FI部分的reading19章节的Managing The Duration Gap with Derivatives Overlay: Swaption and Swaption collar这段课程里讲到,要用derivative manage DB plan 的duration gap,说用swaption时首先是long的头寸,可以long receiver swaption,也可以long payer swaption。为什么强调首先是long 的头寸呢,如果short receiver swaption,不也可以起到降低duration的作用么。