:- dynamic dbugging/0.

% dbug/0 turns on dbug messages.
dbug :- dbugging -> true; assert(dbugging).

% dbug(FormatString) defaults the Arguments to [].
dbug(FormatString) :- dbug(FormatString,[]).

% dbug(FS, Args) prints a message if dbugging is on.
dbug(FormatString,Args) :- 
    dbugging -> format(FormatString,Args); true.

% nodbug/0 turns off dbug messages.
nodbug :- retract(dbugging) -> true; true.

% dbugging is off initially.
:- nodbug.

% shouldnt/0, shouldnt/1, and shouldnt/2 are some simple routines for debugging messages.
shouldnt :-
  format("~nThis shouldnt have happened",[]),
  fail.

shouldnt(String) :-
  format(String,[]),
  fail.

shouldnt(String,Args) :-
  format(String,Args),
  fail.