DU to represent a rank

gistfile1.fs

    /// A card rank.
    type Rank = 
        | Ace | King | Queen | Jack | Value of int
        static member FromString s =
            match s with
            | "A" -> Ace | "K" -> King | "Q" -> Queen | "J" -> Jack 
            | "10" -> Value(10)
            | _ when s.Length = 1 && s.[0] >= '2' && s.[0] <= '9' -> Value(int(s.[0]) - int('0'))
            | _ -> raise (ArgumentException(sprintf "Invalid rank string: %s" s))
        /// The value of the rank for comparison purposes.
        member r. SortValue = 
            match r with
            | Ace -> 14 // Aces high - low-ace cases are handled elsewhere
            | King -> 13
            | Queen -> 12
            | Jack -> 11
            | Value v -> v