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. Calls to debug/3 
and
assertion/1 are 
removed when the code is compiled for optimization unless the Prolog 
flag optimise_debug is set to true.
Using the predicate assertion/1 you can make assumptions about your program explicit, trapping the debugger if the condition does not hold.
Output and actions by these predicates can be configured using hooks to fit your environment. With XPCE, you can use the call below to start a graphical monitoring tool.
?- prolog_ide(debug_monitor).
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 (an atom). This redirects 
debug information on this topic to the given output. On Linux systems 
redirection can be used to make the message appear, even if the user_error 
stream is redefined using
?- debug(Topic > '/proc/self/fd/2').
A platform independent way to get debug messages in the current 
console (for example, a swipl-win window, or login using ssh 
to Prolog running an SSH server from the libssh pack) is to 
use:
?- stream_property(S, alias(user_error)), debug(Topic > S).
Do not forget to disable the debugging using nodebug/1 before quitting the console if Prolog must remain running.
[search(String)] or a normal option list. Defined options 
are:
true) or inactive (false).
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.