Thanks. Unfortunately, the problem is that a C program (again, unless you don't mind making it invalid) can't start with a (; it must start with a keyword after preprocessing, and before preprocessing, the only ways around that are by starting with #, %:, //, /*, or _Pragma, possibly combined with \. (Technically, trigraphs could also be used, but ?? causes even bigger problems.) Comments in Lisp start with ; or are embedded in #|...|#, so the opening must be a valid Lisp expression. The two things that don't break C that I could find are //, and #\ followed by a newline. Both seem to unconditionally break PostScript, but if you have a way around that, I'd be very interested.