HeadTailForRos.cs

//Head+Tail for ROS<T>
static void Deconstruct<T>(this ReadOnlySpan<T> span, out T head, out ReadOnlySpan<T> tail)
{
    switch (span.Length)
    {
        case 0:
            head = default;
            tail = default;
            return;
        case 1:
            head = span[0];
            tail = default;
            return;
        default:
            head = span[0];
            tail = span.Slice(1, span.Length - 1);
            return;
    }
}

static int Sum(ReadOnlySpan<int> list) => list switch
{
    { Length: 0 } => 0,
    var (head, tail) => head + Sum(tail),
};