qselect(xs,k) =
    ~xs -> {
        pivot <- xs[0]!
        left <- xs[#0 <= pivot]
        right <- xs[#0 > pivot]
    } ->
        if k > |left| + 1 then qselect(right, k - |left| - 1)
        else if k == |left| + 1 then [pivot]
        else qselect(left, k);

_search(xs, k) =
    if xs[1] == k then xs
    else if xs[1] > k then _search(xs[0], k)
    else _search(xs[2], k);

sorted(xs) = sorted(xs[0]) ++ [xs[1]] ++ sorted(xs[2]);
search(xs, k) = |_search(xs, k)| != 0;
insert(xs, k) = _insert(k, _search(xs, k));
_insert(k, xs) = if |xs| == 0 then xs << [] << k << [] else xs