[Chapter 4.12] The following excerpt is taken from an article titled Denver Investment to Make $800 Million Treasury Move, that appeared in the December 9, 1991, issue of BondWeek, p. 1:Denver Investment Advisors will swap $800 million of long zero-coupon Treasuries for intermediate Treasuries. . . . The move would shorten the duration of its $2.5 billion fixed-income portfolio. . . .Why would the swap described here shorten the duration of the portfolio?