Performing an action only during backtracking

Help!
1

Here is a silly code example about the Search for the Holy Grail.

The Knights ride forth to find the Grail. A Knight needs to press forward 6 times before the Grail is found. At each forward move, the Knight may encounter the misfortune of dying with p(1/3).

?- acquire_grail.
gavain rides forth!
gavain: 'Arrgh!'
lancelot rides forth!
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'Arrgh!'
perceval rides forth!
perceval: 'The search continues!'
perceval: 'The search continues!'
perceval: 'The search continues!'
perceval: 'The search continues!'
perceval: 'Arrgh!'
Camelot is finished!
false.

More optimistic:

?- acquire_grail.
gavain rides forth!
gavain: 'Arrgh!'
lancelot rides forth!
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'God be praised!'
The Holy Grail has been found by brave lancelot!
Here is how to get there: [fwd,fwd,fwd,fwd,fwd,fwd]
true.

I want to announce the death of a Knight during backtracking, when going from format/2 back to member/2 in acquire_grail/0, in a natural way.

What’s the simplest way?

% ===
% Acquiring the Holy Grail is our only hope
% ===

acquire_grail :- 
   knights(Knights),
   member(Knight,Knights),
   %
   % Printing a death announcement prior to getting the next knight
   % online is a bit hard due to backtracking! How do I do that simply?
   %
   format("~q rides forth!\n",[Knight]),
   search_for_the_grail(Knight,[],Map),
   !, % Victory Cut!
   announce(found,Knight,Map).
                        
acquire_grail :- 
   announce(its_all_over,_,_),fail.

% ===
% The King's Messenger proclaims
% ===
   
announce(found,Who,Map) :- 
  format("The Holy Grail has been found by Brave Knight ~q!\n",[Who]),
  format("Here is how to get there: ~q\n",[Map]).
  
announce(its_all_over,_,_) :- 
  format("Camelot is finished!\n").

% ===
% These Knights hang around the round table usually
% ===

knights([gavain,lancelot,perceval]).

% ===
% The Holy Grail is found after pressing forward 6 times
% But at each move, the Knight has 1/3 chance of dying 
% ===

search_for_the_grail(Knight,Acc,Acc) :-    
   length(Acc,6),
   !, % guard cut
   format("~q: 'God be praised!'\n",[Knight]).
   
search_for_the_grail(Knight,Acc,Map) :- 
   random(0.0,1.0,X), 
   X < 0.666,             
   !, % guard cut
   format("~q: 'The search continues!'\n",[Knight]),
   search_for_the_grail(Knight,[fwd|Acc],Map). 
      
search_for_the_grail(Knight,_,_) :- 
  format("~q: 'Arrgh!'\n",[Knight]),
  fail.
2

Simplest way?
Edit: Technically this isn’t during backtracking, so just putting the dies message after the Arrgh message would accomplish the same thing.

acquire_grail :-
   knights(Knights),
   nb_delete(knight),
   member(Knight,Knights),
   ignore(nb_current(knight,K) -> format("~w dies!~n",[K])),
   nb_setval(knight,Knight),

?- acquire_grail.
gavain rides forth!
gavain: 'The search continues!'
gavain: 'The search continues!'
gavain: 'Arrgh!'
gavain dies!
lancelot rides forth!
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'Arrgh!'
lancelot dies!
perceval rides forth!
perceval: 'Arrgh!'
Camelot is finished!

?- acquire_grail.
gavain rides forth!
gavain: 'The search continues!'
gavain: 'Arrgh!'
gavain dies!
lancelot rides forth!
lancelot: 'The search continues!'
lancelot: 'The search continues!'
lancelot: 'Arrgh!'
lancelot dies!
perceval rides forth!
perceval: 'The search continues!'
perceval: 'The search continues!'
perceval: 'The search continues!'
perceval: 'The search continues!'
perceval: 'The search continues!'
perceval: 'The search continues!'
perceval: 'God be praised!'
The Holy Grail has been found by Brave Knight perceval!
Here is how to get there: [fwd,fwd,fwd,fwd,fwd,fwd]
true.
3

If you want to perform some extra during redo. Try this:

?- member(X, [1,2,3]), (true; write(extra), nl, fail).

Here is a run:

?- member(X, [1,2,3]), (true; write(extra), nl, fail).
X = 1 ;
extra
X = 2 ;
extra
X = 3 ;
extra
false.