code: func reverseIndexRune(s string, r rune) int {
  	if s == "" {
  		return -1
  	}
  
  	rs := []rune(s)
  	for i := len(rs) - 1; i >= 0; i-- {
  		if rs[i] == r {
  			return i
  		}
  	}
  
  	return -1
  }
docs: // reverseIndexRune finds the last rune r in s, returning -1 if not present.