type Stack =
| Empty
| Stack of String list

let push item deck =
    match deck with
    | Empty -> Stack [item]
    | Stack d -> Stack (item::d)

let pop (Stack d) =
    match d with
    | h::[] -> h,Empty
    | h::t -> h,(Stack t)

let empty = Empty
let s,st = pop empty

let pop stack =
    match stack with
    | Empty -> None, Empty
    | Stack (h::[]) -> Some h,Empty
    | Stack (h::t) -> Some h,(Stack t)

let pop stack =
    match stack with
    | Empty -> Error "Empty Stack"
    | Stack (h::[]) -> Ok (h,Empty)
    | Stack (h::t) -> Ok (h,(Stack t))