This is a good explanation of Lisp macros vs C preprocessor. But how do macros ensure an open future for Lisp while dooming C to strange limitations? In both cases the user is simply telling a compiler to do something and the compiler is using a fixed list of instructions to pass the message onto the machine. It would seem the limiting factor in both cases is the compiler or the machine architecture. Would Lisp be capable of generating its own architecture via HDL and its own instruction set at runtime?
Regardless, let us assume PROGN is what makes the difference. What real world task could be performed with PROGN that would be impossible to preform with C?