code: def ai(board, who='x'):
      
      return sorted(board.possible(), key=lambda b: value(b, who))[-1]
docs: Returns best next board
  
      >>> b = Board(); b._rows = [['x', 'o', ' '], ['x', 'o', ' '], [' ', ' ', ' ']]
      >>> ai(b)
      < Board |xo.xo.x..| >