/usr/local/lib/swipl/library/prolog_stack.pl
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 prolog_stack:stack_guard(+PI) is semidet[multifile, library(prolog_stack)]
Dynamic multifile hook that is normally not defined. The hook is called with PI equal to none if the exception is not caught and with a fully qualified (e.g., Module:Name/Arity) predicate indicator of the predicate that called catch/3 if the exception is caught.

The exception is of the form error(Formal, ImplDef) and this hook succeeds, ImplDef is unified to a term context(prolog_stack(StackData), Message). This context information is used by the message printing system to print a human readable representation of the stack when the exception was raised.

For example, using a clause stack_guard(none) prints contexts for uncaught exceptions only. Using a clause stack_guard(_) prints a full stack-trace for any error exception if the exception is given to print_message/2. See also library(http/http_error), which limits printing of exceptions to exceptions in user-code called from the HTTP server library.

Details of the exception decoration is controlled by two Prolog flags:

backtrace_depth
Integer that controls the maximum number of frames collected. Default is 20. If a guard is specified, callers of the guard are removed from the stack-trace.
backtrace_show_lines
Boolean that indicates whether the library tries to find line numbers for the calls. Default is true.
 stack_guard(+Reason) is semidet[multifile, library(prolog_stack)]
Dynamic multifile predicate. It is called with none, 'C' or the predicate indicator of the guard, the predicate calling catch/3. The exception must be of compatible with the shape error(Formal, context(Stack, Msg)). The default is to catch none, uncaught exceptions. 'C' implies that the callback from C will handle the exception.