%let Mv=shoes;
%macro PRODUCT(Mv=bicycles);
%let Mv=clothes;
%mend;
%PRODUCT(Mv=tents)
%put Mv is &Mv;
结果是 Mv is shoes;
%let Mv=shoes;
%macro PRODUCT(Mva=bicycles);
%let Mv=clothes;
%put aa;
%mend;
%PRODUCT(Mva=tents)
%put Mv is &Mv;
结果是Mv is clothes;
求结果分析,为何