In reply to Rick Wicklin.

Thanks Rick for clarification. Yes, the code is short, but more clarification for its complexity is needed.
SAS provide CALL EXECUTE(arugment) is very helpful to extend program logic at runtime. Here argument is a string constant or variable with SAS MACRO or SAS Statement.
However, string quoted with single quote mark ' and double quote mark " have different logic:

If argument is quoted with double quote mark ", the argument is parsed before current data step compliation. if it is quoted with single quote mark ', the argument is parsed during program execution. Anyway, MACRO code in argument always being triggered when argument is parsed (before current DATA Step or at CALL EXECUTE statement), while non-MACRO code always be triggered after current DATA step code execution finished.