数据步中的宏怎样发生作用的？

朱世武金融建模的这段里面的宏是怎样被调用，怎样起作用的？求大神指教options nodate nonotes nosource; /*不输出时间、注释和源程序到日志LOG上 */data ResDat.lg_shanghai(keep=date);set ResDat.Idx000001; where 1995<=year(date)<=2005; /* 全部交易日期通过上证指数的行情取得 */%macroa(x,y);/*求日对数收益率*/data a(keep=date r_1);set ResDat.stk&x;where 1995<=year(date)<=2005; adjclpr=clpr*Mcfacpr;/*用调整后的股价计算，Mcfacpr为累积股价调整乘子 */r_1=log(adjclpr)-log(lag(adjclpr));

/*将所求的收益率合并到数据集ResDat.lg_shanghai中 */dataResDat.lg_shanghai(rename=(r_1=r&x));merge ResDat.lg_shanghai a;by date;data ResDat.lg_shanghai;set ResDat.lg_shanghai;if r&x=. then r&x=0;else r&x= r&x;%mend a;%include "SHStk.txt";run;

上面有点乱，其实就是这个，我稍稍改了一点
data lg_shanghai(keep=date);
  set d.idx000001;
  where 1995<=year(date)<=2005;
  %macro a(x,y);
  data a(keep=date r_1);
    set d.stk&x.;
        where 1995<=year(date)<=2005;
        adjclpr=clpr*Mcfacpr;
        r_1=log(adjclpr)-log(lag(adjclpr));
        data lg_shanghai(rename=(r_1=r&x));
          merge lg_shanghai a;
          by date;
        data lg_shanghai;
          set lg_shanghai;
          if r&x=. then r&x=0;
          else r&x=r&x;
        %mend a;
        %include "e:\SHSTK.txt";
run;