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The predicate flag/3 is the oldest way to store global non-backtrackable data in SWI-Prolog. Flags are global and shared by all threads. Their value is limited to atoms, small (64-bit) integers and floating point numbers. Flags are thread-safe. The flags described in this section must not be confused with Prolog flags described in section 2.12.
next_id(Id) :- flag(my_id, Id, Id+1).