code: func NewBoard(size int) (*Board, error) {
  	b := &Board{
  		size:  size,
  		tiles: map[*Tile]struct{}{},
  	}
  	for i := 0; i < 2; i++ {
  		if err := addRandomTile(b.tiles, b.size); err != nil {
  			return nil, err
  		}
  	}
  	return b, nil
  }
docs: // NewBoard generates a new Board with giving a size.