%macro test( VarX, M_VarY);

/* VarX is a normal macro variable, so itis passed by

  value. */

/* Modify the value of VarX */

%let VarX=Customer_Income;

%put ********** Inside the macro test**************;

%put VarX=&VarX;

/* M_VarY is initialized as null and willbe changed

  here to return the value "Scorecard".  */

%put Initial value of M_VarY=&M_VarY;

%let &M_VarY=Scorecard;

%put ********* Leaving macro test******************;

%mend;

/* Using the macro test */

/* switch off the code echo */

option nosource;

%let VarX=Customer_Age; /* value of varX */

%let VarY=;    /* initializing VarY to a null string */

/* display the values before calling themacro */

%put ********  before calling the macro ************;

%put VarX=&VarX;

%put VarY=&VarY;

%test(&VarX, VarY);   /* Note that VarY is used without & */

/* display the values after executing themacro */

%put ********  after executing the macro ***********;

%put VarX=&VarX;

%put VarY=&VarY;

关于M_定义的宏参数，在其中的红字行，为什么在M_VarY=Scorecard前面用一个&？

请问有没有教材具体介绍这个的，《SAS统计分析使用宝典》《The Little SAS Book》这两本书都没有介绍这个问题。