let fib5 (n : int64) (m : int64) = 
    let mutable dead: List<int64> = [1L; 1L]
    let mutable last = 0L
    let mutable next = 1L
    seq {
        1L
        for i in 1L .. (n - 1L) do
            if i < m
            then
                let temp = last + next
                dead <- dead @ [temp]
                last <- next
                next <- temp
                next
            else 
                if i = m
                then
                    let temp = last + next - dead.Item 0
                    dead <- dead.Tail
                    dead <- dead @ [temp]
                    last <- next
                    next <- temp
                    next
                else
                    let temp = last + next - dead.Item 0
                    dead <- dead.Tail
                    dead <- dead @ [temp]
                    last <- next
                    next <- temp
                    next
        }
        |> Seq.toArray