code: func format(a []rune, c *Cursor) string {
  	i := c.Position
  	var b []rune
  
  	out := make([]rune, 0)
  	if i < len(a) {
  		b = c.Cursor([]rune(a[i : i+1]))
  		out = append(out, a[:i]...)   // does not include i
  		out = append(out, b...)       // add the cursor
  		out = append(out, a[i+1:]...) // add the rest after i
  	} else {
  		b = c.Cursor([]rune{})
  		out = append(out, a...)
  		out = append(out, b...)
  	}
  	return string(out)
  }
docs: // insert the cursor rune array into r before the provided index