If macros are running at compile time, then we don't think that this particular code is self modifying. If we run code in a Lisp interpreter, then one can writ

If macros are running at compile time, then we don't think that this particular code is self modifying. If we run code in a Lisp interpreter, then one can write self-modifying code. It's possible to get visible effects. It's also possible to use it during debugging code.