code: func countUniqueChars(s string) int {
  	m := make(map[rune]struct{})
  
  	for _, c := range s {
  		c = unicode.ToLower(c)
  		if _, ok := m[c]; !ok {
  			m[c] = struct{}{}
  		}
  	}
  
  	return len(m)
  }
docs: // countUniqueChars returns the amount of unique runes in a string