[skissane]

Common Lisp is another language in which this should be possible. Common Lisp allows you to run arbitrary code at compile-time, and that code is allowed to modify the language syntax (*READTABLE*, SET-MACRO-CHARACTER, etc). So code could make itself syntactically invalid on Fridays by changing the language syntax depending on the day of the week.

This goes beyond mere Lisp macros, in that ordinary Lisp macro invocations still look like Lisp lists, while with this you can make arbitrary changes to the syntax, you could even make Common Lisp look like Pascal (if you really wanted to)

The designers of Scheme intentionally left this feature out (which was also found in some of Common Lisp’s ancestors, such as Maclisp), but some Scheme implementations/descendants included it anyway (as an extension), such as Racket, Guile and Chicken Scheme.