Experimental This hook is called if the virtual machine 
executes a D_BREAK, set using set_breakpoint/4. Clause 
and PC identify the breakpoint. FR and BFR 
provide the environment frame and current choicepoint. Expression 
identifies the action that is interrupted, and is one of the following:
- call(Goal)
- 
The instruction will call Goal. This is generated for nearly 
all instructions. Note that Goal is semantically equivalent 
to the compiled body term, but might differ syntactically. This is 
notably the case when arithmetic expressions are compiled in optimized 
mode (see optimise). 
In particular, the arguments of arithmetic expressions have already been 
evaluated. Thus, A is 3*B, where B 
equals 3 results in a term call(A is 9)if the clause was 
compiled with optimization enabled.
- !
- 
The instruction will call the cut. Because the semantics of metacalling 
the cut differs from executing the cut in its original context we do not 
wrap the cut in call/1.
- :-
- 
The breakpoint is on the neck instruction, i.e., after 
performing the head unifications.
- exit
- 
The breakpoint is on the exit instruction, i.e., at the end of 
the clause. Note that the exit instruction may not be reached due to 
last-call optimisation.
- unify_exit
- 
The breakpoint is on the completion of an in-lined unification while the 
system is not in debug mode. If the system is in debug mode, inlined 
unification is returned as call(Var=Term).187This 
hack will disappear if we find a good solution for applying D_BREAK to 
inlined unification. Only option might be to place the break on both the 
unification start and end instructions.
If prolog:break_hook/6 succeeds, it must unify Action with 
a value that describes how execution must continue. Possible values for
Action are:
- continue
- 
Just continue as if no breakpoint was present.
- debug
- 
Continue in debug mode. See debug/0.
- trace
- 
Continue in trace mode. See trace/0.
- call(Goal)
- 
Execute Goal instead of the goal that would be executed.
Goal is executed as call/1, 
preserving (non-)determinism and exceptions.
If this hook throws an exception, the exception is propagated 
normally. If this hook is not defined or fails, the default action is 
executed. This implies that, if the thread is in debug mode, the tracer 
will be enabled (trace) and otherwise the breakpoint is 
ignored (continue).
This hook allows for injecting various debugging scenarios into the 
executable without recompiling. The hook can access variables of the 
calling context using the frame inspection predicates. Here are some 
examples.
- Create conditional breakpoints by imposing conditions 
before deciding the return trace.
- Watch variables at a specific point in the execution. Note that 
binding of these variables can be monitored using attributed 
variables, see section 
7.1.
- Dynamically add assertions on variables using assertion/1.
- Wrap the Goal into a meta-call that traces progress of 
the Goal.