impure!

builtin_aggregate(GeneratorPred, CollectorPred, !Accumulator) :-
    % Save some of the Mercury virtual machine registers
    impure get_registers(HeapPtr, SolutionsHeapPtr, TrailPtr),
    impure start_all_soln_neg_context,

    % Initialize the accumulator
    % /* Mutvar := !.Accumulator */
    impure new_mutvar(!.Accumulator, Mutvar),

    (
        % Get a solution.
        GeneratorPred(Answer0),

        % Check that the generator didn't leave any delayed goals outstanding.
        impure check_for_floundering(TrailPtr),

        % Update the accumulator.
        % /* Mutvar := CollectorPred(MutVar) */
        impure swap_heap_and_solutions_heap,
        impure partial_deep_copy(HeapPtr, Answer0, Answer),
        impure get_mutvar(Mutvar, Acc0),
        impure non_cc_call(CollectorPred, Answer, Acc0, Acc1),
        impure set_mutvar(Mutvar, Acc1),
        impure swap_heap_and_solutions_heap,

        % Force backtracking, so that we get the next solution.
        % This will automatically reset the heap and trail.
        fail
    ;
        % There are no more solutions.
        impure end_all_soln_neg_context_no_more,

        % So now we just need to copy the final value of the accumulator
        % from the solutions heap back onto the ordinary heap, and then we can
        % reset the solutions heap pointer. We also need to discard the trail
        % ticket created by get_registers/3.
        % /* !:Accumulator := Mutvar */
        impure get_mutvar(Mutvar, !:Accumulator),
        impure partial_deep_copy(SolutionsHeapPtr, !Accumulator),
        impure reset_solutions_heap(SolutionsHeapPtr),
        impure discard_trail_ticket
    ).