I had the... pleasure of meeting M4 while working on GNU Bison (a grammar parser generator): after parsing the grammar, it calls M4 to generate the parser (in C

I had the... pleasure of meeting M4 while working on GNU Bison (a grammar parser generator): after parsing the grammar, it calls M4 to generate the parser (in C, C++, Java, D or other), defining some variables. So you get a giant c/c++ file with a parser implementation made of M4 definitions, C preprocessor definitions, includes of generated (lookup tables) or user-written (custom parsing actions) files, and it's absolute hell to debug :D