-- board piece
structural type P = X | O | E

structural type Board = Board P P P P P P P P P

use Board Board
use P O X E
use Optional Some None

orElse a b =
  match a with
    None -> b
    a -> a

b = (Board X O X
           O X X
           O E X)

isWin board =
  same : P -> P -> P -> Optional P
  same a b c = if (a == b) && (a == c) && not (a == E)
               then Some a
               else None
  match board with
    -- vertical top/center/bottom
    -- horizontal left/center/right
    -- diagonal rising/falling
    Board a b c
          d e f
          g h i -> (same a b c)

> isWin b
-- Some 3