code: func (c *Cursor) Backspace() {
  	a := c.input
  	i := c.Position
  	if i == 0 {
  		// Shrug
  		return
  	}
  	if i == len(a) {
  		c.input = a[:i-1]
  	} else {
  		c.input = append(a[:i-1], a[i:]...)
  	}
  	// now it's pointing to the i+1th element
  	c.Move(-1)
  }
docs: // Backspace removes the rune that precedes the cursor
  //
  // It handles being at the beginning or end of the row, and moves the cursor to
  // the appropriate position.