参考答案有如下解释，但是没看懂。。。。就是说 添加DEBUG后 可以控制执行的过程？具体应用场景是什么，解释有点抽象啊。
INVOKING THE DEBUGGER
Invoking the Data Step debugger is as simple as
adding a parameter to the DATA statement:
DATA dsname / DEBUG ;
When SAS encounters the DEBUG parameter on
the DATA statement, it changes the default
behavior for processing the Data step.
Normally, the user has no control over the execution
of the code that has been compiled. The default
behavior is to execute the entire Data step program
with no external access. Historically, the only
means available to view or modify the execution of
a Data step has been with hard coded PUT
statements or conditional logic. However, with the
DEBUG parameter, SAS first compiles the Data
step and then enters DEBUG mode so the user can
interact with the execution of the Data step. In
DEBUG mode, the user can view and control how
SAS executes the Data step program statements.