This library is a replacement for format/3 for printing debug messages. Messages are assigned a topic. By dynamically enabling or disabling topics the user can select desired messages. Debug statements are removed when the code is compiled for optimization.
See manual for details. With XPCE, you can use the call below to start a graphical monitoring tool.
?- prolog_ide(debug_monitor).
Using the predicate assertion/1 you can make assumptions about your program explicit, trapping the debugger if the condition does not hold.
debugging(+Topic) may be 
used to perform more complex debugging tasks. A typical usage skeleton 
is:
      (   debugging(mytopic)
      ->  <perform debugging actions>
      ;   true
      ),
      ...
The other two calls are intended to examine existing and enabled debugging tokens and are typically not used in user programs.
nodebug(_) removes 
all topics. Gives a warning if the topic is not defined unless it is 
used from a directive. The latter allows placing debug topics at the 
start of a (load-)file without warnings.
For debug/1, Topic can be a term Topic > Out, where Out is either a stream or stream-alias or a filename (atom). This redirects debug information on this topic to the given output.
thread, time 
or
time(Format), where Format is a format specification for
format_time/3 
(default is %T.%3f). Initially, debug/3 
shows only thread information.user_error, but only prints if Topic is 
activated through
debug/1. Args 
is a meta-argument to deal with goal for the @-command. Output is first 
handed to the hook
prolog:debug_print_hook/3. 
If this fails, Format+Args is translated to text 
using the message-translation (see
print_message/2) 
for the term debug(Format, Args) and then printed to every 
matching destination (controlled by debug/1) 
using print_message_lines/3.
The message is preceded by '% ' and terminated with a newline.
?- prolog_ide(debug_monitor).
assert() macro. It has no effect if Goal 
succeeds. If Goal fails or throws an exception, the following 
steps are taken:
error(assertion_error(Reason, G),_) 
where Reason is one of fail or the exception raised.
fail if Goal 
simply failed or an exception call otherwise. If this hook fails, the 
default behaviour is activated. If the hooks throws an exception it will 
be propagated into the caller of assertion/1.