ALE source-level debugger installation instructions
***The debugger requires SICStus Prolog 3.8.6 or higher***

General:
   - The source_info flag can be switched on either by loading a Prolog file
     in major mode *before* starting the inferior process, or by typing:

     ?- prolog_flag(source_info,_,on).

     at the Prolog prompt after the inferior process has been started.
   - Do not turn on Prolog tracing to use the ALE source-level debugger unless 
     you want to trace through the ALE debugger itself (believe me, it's an
     overrated experience :-)).  When creeping, the ALE debugger will 
     automatically turn on tracing when passing control to a Prolog hook 

XEmacs only:
 - Run XEmacs, load dot.emacs and follow the 3 steps therein:
  A. paste all of the code in dot.emacs into your .emacs file.
     If you insert it after code that contains an error, XEmacs may not
     use it!
  B. enter the directory where the current SICStus versions of prolog.el and
     sicstus-support.el can be found between the pair of double-quotes on the
     indicated line. This is usually the root directory of the SICStus 
     distribution. The ALE source-level debugger will not work properly 
     with versions of prolog.el distributed before release 3.7.
  C. you may modify the port arrows to whatever you wish for call, exit,
     ndexit, redo, fail and exception.  dot.emacs contains their default
     values.  The ALE source-level debugger uses the arrow for null at
     all ports, since some ALE predicates are typically not indented.

 - You will need to restart XEmacs before using the debugger.
 - To use the debugger with XEmacs, you must run SICStus Prolog inside
   XEmacs as an inferior process with the source_info flag set to on.  The
   XEmacs command for running Prolog as an inferior process is
   M-x run-prolog.  This will try to run SICStus Prolog with the command,
   sicstus.  If the command to run SICStus Prolog is something else, you
   must set the environment variable, EPROLOG, to that command in the shell
   that runs XEmacs.
