To use the OPCODEMENU package:

load the file OPCODEMENU and evaluate (MAKE.OPCODE.MENU).  You will be prompted to create a region for the opcode menu.  Do so.

The small "Debugging" menu at the top turns careful mode on or off -- when turned on Prolog runs at half-speed.  Resuming the Lisp system from a LOGOUT automatically turns careful mode on, so be careful.