Type Alias FlatArray<Arr, Depth>

FlatArray: {
    done: Arr;
    recur: Arr extends ReadonlyArray<infer InnerArr>
        ? FlatArray<
            InnerArr,
            [
                -1,
                0,
                1,
                2,
                3,
                4,
                5,
                6,
                7,
                8,
                9,
                10,
                11,
                12,
                13,
                14,
                15,
                16,
                17,
                18,
                19,
                20,
            ][Depth],
        >
        : Arr;
}[Depth extends -1 ? "done" : "recur"]