code: func (w *Window) setCell(x, y int, c Cell) {
  	c.dirty = true
  	pos := x + (y * w.x)
  	if pos < len(w.backingStore) {
  		w.backingStore[pos] = c
  	}
  }
docs: // setCell sets the content of the window cell at the x and y coordinate.
  // setCell shall be called from queue context.