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Predicate flag/3
Availability:built-in
Sourceflag(+Key, -Old, +New)
True when Old is the current value of the flag Key and the flag has been set to New. New can be an arithmetic expression. The update is atomic. This predicate can be used to create a shared global counter as illustrated in the example below. 
next_id(Id) :-
    flag(my_id, Id, Id+1).

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LogicalCaptain said (2020-05-22T10:46:54):0 upvotes 0 0 downvotes
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From the Database page:

"flag/3 is an old SWI-Prolog specific predicate that should be considered deprecated, although there is no plan to remove it."