> byterun/fix_code.c refers to STOP as the instructions with the highest 
> opcode all over the place.  Yet, byterun/instruct.h list EVENT and
> BREAK after STOP.  Could someone illuminate?

My pleasure.  The EVENT and BREAK opcodes never occur in a bytecode
executable file.  They are inserted by the debugger by run-time
modification of the bytecode.  Hence, the program loading code in 
byterun/fix_code.c always sees opcodes that are <= STOP.

- Xavier Leroy