:- pred fib_acc(int::in, int::in, int::in, int::in, int::out) is det.

fib_acc(N, Limit, Prev2, Prev1, Res) :-
    ( N < Limit ->
        % limit not reached, continue computation.
        ( N =< 2 ->
            Res0 = 1
        ;
            Res0 = Prev2 + Prev1
        ),
        fib_acc(N+1, Limit, Prev1, Res0, Res)
    ;
        % Limit reached, return the sum of the two previous results.
        Res = Prev2 + Prev1
    ).