code: func (g *Gui) Rune(x, y int) (rune, error) {
  	if x < 0 || y < 0 || x >= g.maxX || y >= g.maxY {
  		return ' ', errors.New("invalid point")
  	}
  	c := termbox.CellBuffer()[y*g.maxX+x]
  	return c.Ch, nil
  }
docs: // Rune returns the rune contained in the cell at the given position.
  // It checks if the position is valid.