SWI-Prolog -- nb

Availability:built-in

nb_setval(+Name, +Value)

Associates a copy of Value created with duplicate_term/2 with the atom Name. Note that this can be used to set an initial value other than [] prior to backtrackable assignment. Starting with version 9.3.18, if the new Value contains the old value, the old value is not copied. This implies that push/1 below has complexity O(1), regardless of the length of the list Old.

push(Var, Value) :-
    nb_getval(Var, Old),
    nb_setval(Var, [Value|Old]).