code: func (s *stringSet) clean(isSuffix bool) {
  	t := *s
  	if isSuffix {
  		sort.Sort((*bySuffix)(s))
  	} else {
  		sort.Sort((*byPrefix)(s))
  	}
  	w := 0
  	for _, str := range t {
  		if w == 0 || t[w-1] != str {
  			t[w] = str
  			w++
  		}
  	}
  	*s = t[:w]
  }
docs: // clean removes duplicates from the stringSet.