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yap-6.3/Logtalk/examples/points/point_history.lgt

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:- category(point_history).
:- info([
version is 1.0,
date is 1998/3/23,
authors is 'Paulo Moura',
comment is 'Point position history management predicates.',
source is 'Example adopted from the SICStus Objects documentation.']).
:- public(add_to_history/1).
:- mode(add_to_history(+nonvar), one).
:- public(init_history/1).
:- mode(init_history(+list), one).
:- public(history/1).
:- mode(history(-list), zero_or_one).
:- public(print_history/0).
:- mode(print_history, zero_or_one).
:- private(history_/1).
:- dynamic(history_/1).
:- mode(history_(-list), zero_or_one).
add_to_history(Location) :-
::retract(history_(History)),
::assertz(history_([Location| History])).
init_history(History) :-
::retractall(history_(_)),
::assertz(history_(History)).
history(History) :-
::history_(History).
print_history :-
::history_(History),
write('location history: '),
write(History),
nl.
:- end_category.