let rank_of_string (s: string) : rank option =
match s with
| "One" -> Some One
| "Two" -> Some Two
| "Three" -> Some Three
| "Four" -> Some Four
| "Five" -> Some Five
| "Six" -> Some Six
| "Seven" -> Some Seven
| "Eight" -> Some Eight
| "Nine" -> Some Nine
| "Ten" -> Some Ten
| "Jack" -> Some Jack
| "Queen" -> Some Queen
| "King" -> Some King
| "Ace" -> Some Ace
| _ -> None