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beeon backtracking despite the fact that
beeis already retracted.80Example by Jan Burse.
:- dynamic insect/1. insect(ant). insect(bee). ?- ( retract(insect(I)), writeln(I), retract(insect(bee)), fail ; true ). ant ; bee.
If multiple threads start a retract on the same predicate at the same
time their notion of the entry generation is adjusted such that
they do not retract the same first clause. This implies that, if
multiple threads use
once(retract(Term)), no two threads
will retract the same clause. Note that on backtracking over retract/1,
multiple threads may retract the same clause as both threads respect the
logical update view.