给个例子没看明白

比如说，%macro(idx)；
         %if &idx in ('subject_1','subject_2') %then ......;%else ......;
        %mend;
我希望执行%macro(subject_1)和%macro(subject_3)时用'subject_1'或'subject_3'这两个字段名来判断条件。但是这样写，它会把subject_1或subject_3的值（数值型）放在这个if句中进行运算。而如果写'&idx',也只是把字符型的数字代进去而已。我想知道应该怎么写才正确。谢谢

options mprint mlogic symbolgen;

%macro test(idx=);

  %if &idx = s1 or &idx = s2 %then %put Hello world!;
   %else %put The error;
%mend;

%test(idx=s1)