Use of Active Patterns to convert integer to Roman numerals

open System

let rep times letter = [for _ in 1 .. times -> letter]

let div n divisor letter =
    let q = n / divisor
    if q > 0 then 
        let roman = rep q letter
        let rem = n % divisor
        Some(roman,rem)
    else
        None
        
let sub n divisor letterList = if n / divisor > 0 then Some(letterList,n - divisor) else None
        
let (|Thousand|_|) n = div n 1000 'M'
let (|NineH|_|) n    = sub n 900 ['C';'M']
let (|FiveH|_|) n    = div n 500 'D'
let (|FourH|_|) n    = sub n 400 ['C';'D']
let (|Hundred|_|) n  = div n 100 'C'
let (|Ninety|_|) n   = sub n 90 ['X';'C']
let (|Fifty|_|) n    = div n 50 'L'
let (|Forty|_|) n    = sub n 40 ['X';'L']
let (|Ten|_|) n      = div n 10 'X'
let (|Nine|_|) n     = sub n 9 ['I';'X']
let (|Five|_|) n     = div n 5 'V'
let (|Four|_|) n     = sub n 4 ['I';'V']
let (|One|_|) n      = div n 1 'I'

let toRoman (n:int) =

    let rec loop acc dvnd =
        match dvnd with
        | 0                    -> String(acc |> List.rev |> List.collect (fun x->x) |> List.toArray)
        | Thousand (roman,rem) 
        | NineH (roman,rem)
        | FiveH (roman,rem)
        | FourH (roman,rem)
        | Hundred (roman,rem)
        | Ninety (roman,rem)
        | Fifty (roman,rem)
        | Forty (roman,rem)
        | Ten (roman,rem)
        | Nine (roman,rem)
        | Five (roman,rem)
        | Four (roman,rem)
        | One (roman,rem) -> loop (roman::acc) rem
        | _               -> failwithf "unable to convert %A" dvnd
        

    loop [] n
    
toRoman 3999         //"MMMCMXCIX" 
toRoman 3000         //"MMM"
toRoman 495          //"CDXCV"
toRoman 1            // "I"
toRoman 2            // "II"